AGRICULTURAL  GEOLOGY 


BY 


FREDERICK  V.  EMERSON,  Ph.D. 

^rofessor  of  Geology  and  Geologist  for  The  State  Experiment  Station, 
Louisiana  State  University 


NEW  YORK 

JOHN  WILEY  &  SONS,  INC. 

LONDON:   CHAPMAN  &  HALL,  LIMITED 

1920 


COPYRIGHT,  1920,  BY 
HELEN  L.  EMERSON 


PRESS    OP 
BRAUNWORTH  &   CO. 
BOOK   MANUFACTURERS 
BROOKLYN,    N.    V. 


PREFACE 


BECAUSE  of  the  death  of  my  husband,  it  is  not  possible  to  acknowl- 
edge all  the  helpful  suggestions  and  criticisms  offered  by  his  various 
scientific  friends,  but  I  recall  that  he  considered  the  suggestions  of 
Doctor  Heinrich  Ries,  of  Cornell  University,  as  very  valuable,  and  that 
Professor  L.  E.  Call,  of  Kansas  State  Agricultural  College,  and  Professor 
A.  F.  Kidder,  of  Louisiana  State  University,  both  read  several  chapters 
and  made  helpful  suggestions  and  criticisms.  There  were,  however, 
other  friends,  whose  names  I  do  not  know,  who  rendered  similar  ser- 
vices. 

I  believe  the  readers  of  this  book  will  be  interested  to  know  that 
Doctor  Heinrich  Ries  has  kindly  consented  to  aid  in  any  revisions  that 
may  become  necessary. 

H.  L.  EMERSON. 
EAST  PROVIDENCE,  R.  I.,  April,  1920. 


424457 


FOREWORD 


GEOLOGY  AND  AGRICULTURE  are  closely  related,  indeed  it  is  due  to 
geological  processes  that  the  hard  rocks  are  broken  down  to  soil,  and 
essential  mineral  substances  set  free  which  in  some  cases  affect  the 
physical  qualities  of  the  derived  soil,  and  in  others  serve  as  sources  of 
plant  food. 

It  therefore  follows  that  the  student  of  agriculture  should  have  at 
least  an  elementary  knowledge  of  the  processes  and  principles  of  Geology, 
with  especial  reference  to  the  geology  of  soils  and  fertilizers. 

With  this  object  in  view  the  late  Professor  Emerson  prepared  the 
accompanying  work,  but  unfortunately  his  untimely  death  prevented 
his  seeing  it  through  the  press,  the  labor  of  this  devolving  on  Mrs. 
Emerson. 

The  subject  matter  and  mode  of  treatment  are  the  outgrowth  of 
some  years  of  experience  in  teaching  geology  to  agricultural  students, 
and  while  Professor  Emerson  prepared  the  work  primarily  for  class- 
room use,  it  was  also  his  hope  that  it  might  prove  serviceable  for  reading 
and  correspondence  classes. 

On  this  account  he  endeavored  to  make  the  treatment  as  untechnical 
as  possible,  without  sacrificing  scientific  accuracy. 

Those  who  desire  to  follow  the  subject  in  greater  detail  can  do  so 
with  the  aid  of  the  appended  bibliographies  and  lists  of  soil  and  geological 
maps.  Professor  Emerson  also  gave  considerable  attention  to  the 
selection  of  illustrations,  choosing  them  with  the  purpose  of  showing 
specific  items  on  phenomena. 

H.  RIBS. 
ITHACA,  N.  Y.,  April,  1920. 

' 


iv 


CONTENTS 


PAGE 

INTRODUCTION 1 


CHAPTER  I 
MINERALS 5 

General  Characters  of  Minerals,  6;  Color,  6;  Luster,  6;  Streak,  6;  Hard- 
ness, 6;  Tenacity,  7;  Cleavage,  7;  Fracture,  7;  Crystal  Form,  8;  Spe- 
cific Gravity,  8;  Important  Soil  and  Rock-making  Minerals,  8;  Apatite,  8; 
Calcite,  8;  Dolomite,  9;  Gypsum,  9;  Halite,  9;  Nitre,  10;  Kainite,  10; 
Trona,  10;  Mirabilite,  10;  Iron  Minerals,  10;  Hematite,  10;  Limonite,  10; 
Magnetite,  11;  Siderite,  11;  Pyrite,  11;  Silica  and  the  Silicates,  11; 
Feldspars,  12;  Orthoclase,  12;  Plagioclase  Feldspars,  13;  Micas,  13; 
Muscorite,  13;  Biotite,  13;  Olivine,  13;  Hornblende,  13;  Augite,  13; 
Secondary  Silicates,  14;  Zoolites,  14;  Talc,  14;  Glauconite,  14;  Kaolinite, 
14;  Ferromagnesian  Minerals,  15;  References  on  Minerals,  15.  , 

CHAPTER  II 
ROCKS 16 

Mantle  Rock,  16;  Classification,  16;  Igneous  Rocks,  17;  Composition, 
17;  Texture,  18;  Classification,  20;  Descriptions  of  Igneous  Rocks,  20; 
Granitoid  Texture,  20;  Granite,  20;  Syenite,  22;  Diorite,  23;  Gabbro,  23; 
Porphyritic  Texture,  24;  Felsitic  Texture,  25;  'Felsites,  25;  Basalts,  25; 
Glassy  Rocks,  25;  Obsidian,  25;  Pitchstone,  26;  Pumice,  26;  References, 
26;  Occurrences  of  Igneous  Rocks,  27;  Intrusive  Forms,  27;  Dikes,  27; 
Sills,  29;  Volcanic  Necks,  30;  Laccolith,  31;  Stocks  or  Bosses,  31;  Bathy- 
liths,  32;  Vulcanism,  32;  Ejecta  from  Volcanoes,  32;  Lava,  32;  Types  of 
Eruptions,  35;  Fissure  Flows,  36;  Clastic  Rocks,  37;  Agents  Involved  ii 
the  formation  of  Clastic  Rocks,  37;  Sandstones,  39;  Chemical  Composi- 
tion, 40;  Conglomerates,  40;  Shales,  41;  Chemical  Composition,  42; 
Limestone,  42;  Varieties,  44;  Chemical  Composition,  44;  Structure  of 
Sedimentary  Rocks,  45;  Monoclinal  Structure,  45;  References,  46; 
Metamorphic  Rocks,  46;  Changes  Produced  by  Metainorphism,  47; 
Agents  of  Metamorphism,  47;  Heat,  47;  Pressure,  48;  Gases  and  Fluids, 


vi  CONTENTS 

i 

PAGE 

48;  Slaty  Cleavage  and  Schistosity,  49;  Complexity  of  Metamorphism, 
50;  Contact  and  Regional  Metamorphism,  50;  Kinds  of  Metamorphic 
Rocks,  52;  Gneiss,  52;  Schists,  52;  Slates,  53;  Quartzite,  54;  Marble,  55; 
Structures  Common  to  all  Rocks,  56;  Joints,  56;  Structures  Due  to  Fold- 
ing, 57;  Dip  and  Strike,  58;  Anticline  and  Syncline,  59;  Topography 
produced  by  Folding,  61;  Faults,  63;  Effects  of  Faulting,  64;  References 
on  Rocks  and  Folded  Rocks,  65;  References  on  Faults,  66. 

CHAPTER  III 

WEATHERING 67 

Erosion,  67;  Processes  of  Weathering,  67;  Residual  Soils,  68;  Processes 
of  Decomposition,  68;  Decomposition,  68;  Carbonation,  68;  Reference,  69; 
Oxidation,  69;  Hydration,  70;  Solution,  71;  Association  of  Decomposi- 
tion Factors,  72;  References,  73;  Disintegration  and  its  Processes,  73; 
Temperature  Changes,  73;  Rapidity  of  Temperature  Changes,  74;  Soils 
Due  Primarily  to  Disintegration,  74;  Exfoliation,  75;  Other  Factors,  75; 
Freezing  and  Thawing,  76;  Gravity,  77;  References,  77;  Weathering 
Work  of  Plants,  78;  The  Work  of  Roots,  78;  Decay  and  Humification,  78; 
Weathering  Effects  of  Humus,  80;  Reference,  80;  Microorganisms,  80; 
Bacteria,  81;  References,  81;  The  Weathering  Work  of  Animals,  81 J 
References,  82;  Interaction  of  Weathering  Factors,  82;  Rate  of  Weathering, 
83;  References,  84. 

CHAPTER  IV 
RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 85 

Limestone  and  Marble  Soils,  Introductory,  85;  Clay  Soils  from  Lime- 
stone, 86;  Soils  from  Cherty  Limestones,  86;  Soils  from  Dolomitic  Lime- 
stones, 87;  Chemical  and  Mineralogical  Changes,  88;  Topography,  89; 
Notable  Regions,  90;  Reference,  90;  Sandstone  and  Quartzite  Soils, 
Introductory,  90;  The  Weathering  of  Sandstones,  91;  Quartzite  Soils,  91; 
Shale  and  Slate  Soils,  92;  Comparison  of  Sedimentary  Rocks,  94;  Refer- 
ences— Residual  Soils  from  Sedimentary  Rocks,  95;  Granite  and  Gneiss 
Soils,  95;  Weathering  of  Granites  and  Gneisses,  95;  Chemical  and  Min- 
eralogical Changes,  97;  Notable  Regions,  98;  Soils  from  Basic  Rocks — 
Diorite  and  Basalt,  99;  Introductory,  99;  Weathering  in  General,  99; 
Diorites  and  their  Soils,  99;  Basalts  and  their  Soils,  100;  Obsidian  Soils, 
101;  Schist  Soils,  101;  References,  102;  Inherited  Soils,  103;  Reference, 
106. 

CHAPTER  V 
WIND  WORK  AND  EOLIAN  SOILS 107 

Introductory,  107;  Atmospheric  Dust,  107;  Wind  Transportation,  108; 
Dunes,  109;  Wind  Abrasion,  110;  Soil  Blowing,  110;  The  Loess,  112; 


CONTENTS  vii 

PAGE 

Mechanical  Composition,  112;  Mineralogical  Composition,  113;  Origin 
of  Loess,  114;  the  Problem  Stated,  114;  the  Possible  Agents,  115;  Coop- 
erating Agents,  117;  Loessial  Soils,  117;  References  on  Wind  Work  and 
the  Loess,  119. 


CHAPTER  VI 

GROUND  WATER 120 

The  Water  Table,  120;  Ground  Water  Movements,  122;  Work  of  Ground 
Water,  122;  Solution,  122;  Caverns  and  Sink  Holes,  123;  Deposition  by 
Ground  Water,  124;  Mineral  Veins,  124;  Soil  Water,  124;  Movements, 
125;  Capillary  Water,  125;  Mechanical  Work  of  Soil  Water,  126;  Chem- 
ical work  and  Soil  Water,  126;  Oxidation,  126;  Carbonation,  127;  Solu- 
tion, 127;  Deposition,  127;  Hard  Pan,  129;  Alkali,  129;  References  on 
Soil  Water,  130;  Wells  and  Springs,  131;  References,  132. 

CHAPTER  VII 
STREAMS  AND  THEIR  WORK;  ALLUVIAL  SOILS 133 

Sources  of  Streams,  133;  Stream  Organization,  133;  Velocity,  134;  Stream 
Work,  134;  Introductory,  134;  Corrosion,  134;  Corrasion,  135;  The 
Development  of  Valleys  and  Divides,  137;  Incised  Meanders,  138;  Soil 
Erosion,  140;  Factors,  141;  Remedies  for  Soil  Erosion,  142;  Bad  Land 
Topography,  143;  Stream  Transportation,  Factors,  143;  The  Stream 
Load  in  Transit,  145;  Abrasion  of  the  Load  in  Transit,  146;  Stream 
Deposits.  Alluviation,  147;  Factors — Diminished  Velocity — Diminished 
Volume,  148;  The  Load  Itself,  149. 

CHAPTER  VIII 
CLASSES  OF  ALLUVIAL  DEPOSITS 150 

Alluvial  Deposits  in  Channels,  150;  Flood  Plains,  151;  Origin,  151; 
Natural  Levee  and  Back  Lands,  151;  Soils  of  Flood  Plains,  153;  Variability 
of  Alluvial  Soils,  155;  Flood  Plains  and  Valleys,  156;  Flood  Plain  mean- 
ders, 157;  Development  of  Meanders,  158;  Deposition  by  Meanders,  159; 
The  Settlement  of  Flood  Plains,  159;  The  Missisippi  Flood  Plain,  160; 
Alluvial  Terraces,  161 ;  Origin,  161;  Terrace  Soils,  163;  Alluvial  Soils  and 
Stream  Basins,  165;  Deltas,  166;  Growth  of  Deltas,  167;  Classes  of  Deltas, 
169;  Delta  Materials,  170;  Delta  Soil,  171;  Alluvial  Fans  and  Cones,  171; 
Origin,  173;  Favorable  Conditions,  174;  Notable  Regions,  174;  Soils, 
176;  References — Streams  and  Stream  Work,  177;  Soil  Erosion,  177; 
Alluvial  Fans,  178;  Alluvial  Soils,  178;  The  Cycle  of  Erosion,  178;  Youth, 
178;  Maturity,  179;  Age,  179;  Stages  and  Soils,  180;  References,  ISO. 


viii  CONTENTS 

CHAPTER  IX 

PAGE 

SOIL  CREEP.    COLLUVIAL  SOILS 181 

Introductory,  181;  Soil  Creep,  181;  Factors,  181;  Associated  Agents, 
183;  Differential  Movements,  184;  Soil  Creep  and  Rock  Variation,  185; 
Colluvial  Soils,  186;  Talus,  187;  Landslides  and  Avalanches,  188;  Refer- 
ences, 189. 

CHAPTER  X 

GLACIERS  AND  GLACIATION;    GLACIAL  SOILS 190 

Introductory,  190;  Kinds  of  Glaciers,  191;  Mountain  Glaciers,  191; 
Continental  Glaciers,  192;  Conditions  of  Formation,  192;  Movements, 
192;  The  Rapidity  of  Glacial  Movement,  193;  Ice  Advance  and  Retreat, 
193;  References,  194;  The  Work  of  Glaciers,  194;  Introductory,  194; 
Ice  Tools  of  Erosion,  194;  The  Vigor  of  Ice  Erosion,  196;  Plucking,  197; 
The  Ice  Load  and  its  Transportation,  197;  A  Glacier  Acts  as  a  Huge  Mill, 
199;  Erratics,  200;  Glacial  Deposition,  200;  The  Drift,  201;  The  Thick- 
ness of  the  Drift,  201;  Composition  of  Drift,  202;  Moraines,  203;  Ter- 
minal Moraines,  204;  Topography  of  Terminal  Moraines,  204;  Reces- 
sional Moraines,  205;  The  Soils  of  Terminal  and  Recessional  Moraines, 
206;  The  Ground  Moraine,  206;  The  Soils  of  Ground  Moraines,  207; 
Drumlins,  208;  Relations  of  Drift  and  Glacial  Soils  to  Local  Formations, 
208;  Introductory,  208;  Influence  of  Local  Rocks  on  Glacial  Soils,  209;  Ice 
Movement  and  Rock  Strike,  211;  Fluvio-glacial  "Work,  212;  Outwash 
Plains,  213;  Soils  of  Outwash  Plains,  213;  Valley  Train,  216;  Kames  and 
Eskers,  217;  Typical  Area,  218;  Topographic  and  Drainage  Changes  Due 
to  Glaciation,  219;  Features  of  Erosion,  221;  Drainage  Changes,  222 ;  Mar- 
ginal Glacial  Lakes,  223;  Features  of  Abandoned  Glacial  Lakes,  224; 
Soils,  224;  Stages  in  the  Glacial  Period,  228;  Introductory,  228;  Soils 
and  Glacial  Stages,  229;  Stages  in  the  Glacial  Period,  231;  The  Sub-aftonian 
Stage,  232;  Aftonian  Interglacial  Stage,  232;  Kansan  Stage,  232;  Yar- 
mouth Interglacial  Stage,  232;  Illinois  Stage,  232;  Sangamon  Inter- 
glacial  Stage,  233;  lowan  Stage,  233;  Peorian  Interglacial  Stage,  233; 
Early  Wisconsin  and  Late  Wisconsin  Stages,  233;  The  Loess  and  Glacia- 
tion, 234;  Value  of  Glaciation,  234;  Causes  of  the  Glacial  Period,  236; 
General  References  on  Glaciation,  236;  References  on  the  Great  Glacial 
Lakes,  237;  References  on  Glacial  Soils,  237. 

CHAPTER  XI 

LAKES  AND  SWAMPS;  LACUSTRINE  AND  CUMULOSE  SOILS;  LAKES;  LACUSTRINE 

SOILS : 238 

Introductory,  238;  Kinds  of  Lakes,  238;  Glacial  Lakes,  238;  River 
Lakes,  239;  Delta  Lakes,  239;  Coastal  Plain  Lakes,  239;  Effects  of  Lakes, 


CONTENTS  ix 

PAGE 

240;  Shore  Regions  of  Lakes,  240;  Waves,  241;  Barrier  Beaches  or  Off- 
Shore  Bars,  242;  Shore  Lines  at  Different  Water  Levels,  242;  The  Soils 
Associated  with  Lake  Beaches,  243;  Deltas,  245;  Lake  Deposits  and  Lake 
Basins,  246;  Extinction  of  Lakes,  247;  Topography  of  Lake  Bottoms,  247; 
Lacustrine  or  Lake  Made  Soils,  248;  Saline  Lakes,  248;  Swamps,  Cumu- 
lose  Soils,  250;  Factors,  250;  Classes  of  Swamps,  251;  Glacial  Swamps, 
251;  Alluvial  Swamps,  251;  Coastal  Plain  Swamps,  252;  Filling  of  Lakes 
and  Swamps,  253;  Lake  and  Swamp  Deposits,  255;  Peat,  255;  Cumulose 
Soils,  256;  References  on  Lakes  and  Swamps,  258. 


CHAPTER  XII 
OCEANS 259 

Introductory,  259;  Movements,  259;  Shore  Features,  260;  Barrier 
Beaches  and  Lagoons,  260;  Filling  of  a  Lagoon,  260;  References  on  Marine 
Marshes,  262;  Sea  Islands,  262;  Depressed  and  Elevated  Coasts,  Introduc- 
tory, 262;  Depressed  Coasts,  263;  Elevated  Coasts,  263;  Coastal  Plains, 
264;  The  Coastal  Plain  of  North  America,  265;  Origin,  265;  The  Mate- 
rials of  the  Coastal  Plain,  265;  Boundaries,  266;  Erosion  of  the  Coastal 
Plain,  266;  The  Lafayette  and  Columbia  Formations,  267;  Origin  of 
the  Lafayette  and  Columbia  Formations,  268;  References,  269;  Marine 
Deposits,  269;  Deep  Water  Deposits,  269;  Sea  Life,  270;  References,  271. 

CHAPTER  XIII 
MINERAL  FERTILIZERS .•*  : 273 

Phosphates,  Kinds,  273;  Phosphate-bearing  Rocks,  273;  General  Origin, 
274;  Primary  Origin,  274;  Phosphate  Producing  Regions,  275;  The 
Tennessee  Phosphates,  275;  Residual  Phosphate,  275;  Bedded  Rock  Phos- 
phate, 276;  The  Florida  Phosphates,  278;  Land  Pebble  Phosphate,  280; 
River  Pebble  Phosphate,  280;  Ultimate  Sources,  280;  Other  Areas,  280; 
Potash,  281;  The  Stassfurt  Region,  282;  Nitrates,  283;  Gypsum  and 
Limestone,  284;  References,  284. 


CHAPTER  XIV 
SOIL  REGIONS  OF  THE  UNITED  STATES 285 

Introductory,  285;  The  Coastal  Plain,  286;  The  Piedmont  Plateau,  286; 
The  Appalachian  Mountain  and  Plateau  Region,  287;  The  Limestone 
Valleys  and  Uplands,  288;  The  Glacial  and  Loessial  Soil  Regions,  289; 
Great  Plains  Region,  290;  The  Rocky  Mountain  and  Plateau  Region, 
290;  The  Great  Basin,  290;  Arid  Southwest  Region,  291;  The  Pacific 
Region,  291;  References,  292. 


X  CONTENTS, 

CHAPTER  XV 

PAGE 

HISTORICAL  GEOLOGY , 293 

Introduction,  293;  The  Pre-Cambrian  Era,  293;  The  Palezoic  Era,  294; 
Cambrian  Period,  294;  Ordovician  Period,  294;  Silurian  Period,  294; 
Devonian  Period,  295;  Mississippian  Period,  295;  Pennsylvanian  Period, 
295;  Permian  Period,  296;  Mesozoic  Era,  296;  Triassic  and  Jurassic 
Periods,  296;  Comanchean  Cretaceous  Period,  297;  Cenozoic  Era,  297; 
Tertiary  Period,  297;  Quaternary  Period,  298. 

APPENDIX.    SOIL  MAPS.  . .  .  299 


LIST  OF  ILLUSTRATIONS 


FIG.  PAGE 

1.  A  rock  record  of  an  ancient  beach  showing  rain  prints  and  rill  marks 1 

2.  Cleavages  of  Calcite  and  Feldspar 7 

3.  Shell-like  fracture  of  flint 7 

4.  Crystal  forms  of  apatite,  feldspar,  and  garnet 8 

5.  Granite  showing  granitoid  texture 18 

6.  Granitoid  texture.     A  microphotograph 18 

7.  Glassy  textures  of  obsidian 19 

8.  Porphyritic  texture  showing  light-colored  phenocrysts  of  feldspar  embedded 

in  a  dark-colored  mass 19 

9.  Diagram  showing  the  chemical  composition  of  a  biotite  granite 21 

10.  Diagram  showing  the  mineralogical  composition  of  a  biotite  granite 21 

11.  Diagram  showing  the  chemical  composition  of  syenite 22 

12.  Diagram  showing  the  mineralogical  composition  of  a  syenite 22 

13.  Diagram  showing  the  chemical  composition  of  a  diorite 23 

14.  Diagram  showing  the  mineralogcal  composition  of  a  diorite 23 

15.  Diagram  showing  the  chemical  composition  of  a  gabbro 24 

16.  Diagram  showing  the  mineralogical  composition  of  a  gabbro 24 

17.  The  great  dike,  Spanish  Peaks  Region,  Colo 28 

18.  Diagram  of  a  dike  intruded  into  shales ^28 

19.  Diagram  showing  sills  and  dikes 29 

20.  The  Palisades,  a  sill,  N.  Y.     Two  views  and  diagram  of  the  general  structure 

in  the  vicinity 30 

21.  Pilot  Knob,  Texas,  a  volcanic  plug 31 

22.  Sundance  Mountain,  Wyo.,  a  laccolithic  mountain.     Photograph  and  a 

diagram  showing  structure 31 

23.  A  volcanic  bomb 33 

24.  Castle  Rock,  Nebraska,  photograph  shows  white  volcanic  dust 33 

25.  Very  fine  volcanic  dust,  Nebraska.     Magnified 34 

26.  Crater  of  an  extinct  volcano  in  Arizona 34 

27.  A  recent  lava  flow  .in  New  Mexico  showing  "ropy"  appearance  due  to 

unequal  flowage 35 

28.  Map  of  the  Columbia  River  lava  flows 35 

29.  Two  views  of  the  Columbia  lava  plateau,  Washington 36 

30.  Stratified  rocks 38 

xi 


xii  LIST  OF  ILLUSTRATIONS 

FIQ.  PAGE 

31.  Diagram  showing  composite  analysis  of  253  sandstones 40 

32.  Conglomerate • 41 

33.  Diagram  showing  composite  analysis  of  78  shales 42 

34.  Diagram  showing  the  composition  of  pure  kaolinite 42 

35.  A  fossilliferous  limestone 43 

36.  Cherty  limestone 44 

37.  Diagram  showing  composite  analysis  of  345  limestones 44 

38.  Diagram  to  illustrate  the  change  from  bituminous  coal  to  anthracite  by  an 

intrusion  of  lava 48 

39.  Diagram  to  illustrate  the  metamorphism  of  bituminous  coal  to  anthracite 

because  of  folding 48 

40.  Foliated  gneiss  produced  by  intense  folding 49 

41.  Diagram  to  illustrate  the  development  of  schistocity  by  pressure 50 

42.  Diagram  to  illustrate  contact  metamorphism 51 

43.  Diagram  to  show  limestone  metamorphosed  by  an  intrusion  of  lava 51 

44.  Section  of  metamorphosed  rocks  in  the  Green  Mountains,  Mass 52 

45.  Slate  developed  from  shale  by  metamorphism 53 

46.  Microphotograph  of  quartzite 54 

47.  Microphotographs  of  limestone  and  marble 55 

48.  Vertical  and  horizontal  joints  in  granite,  Conn 56 

49.  Generalized  diagram  showing  structure,  topography,  and  soils  of  folded 

rocks  in  northern  Georgia 57 

50.  Diagram  to  illustrate  dip,  strike,  and  outcrop 58 

51.  Diagram  illustrating  the  changing  width  of  outcrop  due  to  variations  in  dip .  58 

52.  Anticline  of  sandstone,  Md 59 

53.  Syncline  of  shale,  Pa 59 

54.  Map  of  part  of  the  Appalachian  Ridge  Belt 60 

55.  Diagram  to  show  rock  structure  and  topography  of  the  Cumberland  Valley 

and  South  Mt.  in  Pa 60 

56.  Diagram  to  illustrate  the  evolution  of  valleys  on  anticlines 61 

57.  Diagram  to  illustrate  the  development  of  ridges  and  valleys  on  folded  rocks.  62 

58.  Diagram  and  photograph  of  ridges  caused  by  folding 62 

59.  Diagram  of  the  Blue  Grass  and  Highland  Rim  regions,  Term 63 

60.  Small  faults,  Texas;  the  strata  do  not  mateh 63 

61.  Diagram  to  illustrate  a  fault 64 

62.  Diagram  showing  an  effect  of  faulting  on  soils 64 

63.  Fault  scarp  in  Arizona 65 

64.  Gradations  from  limestones  below  to  soils  above,  Kansas 68 

65.  Weathering  has  etched  out  delicate  structures  in  limestone 69 

66.  Pitted  limestone  due  to  solution,  Missouri 71 

67.  "Enchanted  Rocks,"  Texas.     The  hills  are  of  granite  and  show  exfoliation 

on  a  large  scale - 76 

68.  Residual  boulders  surrounded  by  soft,  disintegrated  granite 77 

69.  "Lumps"  of  root  bacteria  growing  on  alfalfa  roots 81 

70.  Limestone  and  its  residual  soil 86 

71.  Diagram  to  illustrate  the  topography  and  soils  from  cherty  dolomitic  lime- 

stones, limestones  and  sandstones  and  shales  in  northern  Georgia 87 


LIST  OF  ILLUSTRATIONS  xiii 

FIG.  PAGE 

72.  Weathering  of  cherty  dolomitic  limestone 88 

73.  Diagram  showing  the  compositions  of  fresh  magnesian  limestone  and  its 

residual  clay 88 

74.  Soils  derived  from  limestone  in  the  foreground.     The  ridge  in  the  back- 

ground is  underlain  by  sandstone  and  is  covered  by  a  stony  loam ....     90 

75.  Diagram  to  show  the  occurrence  of  rocks  and  their  derived  soils  on  the  Pied- 

mont in  Pennsylvania 92 

76.  Residual  soils  from  slate,  diorite,  and  granite,  North  Carolina 94 

77.  Generalized  diagram  to  show  the  composition  of  limestones,  shales  and 

sandstones 94 

78.  Residual  soils  from  sedimentary  rocks,  Kansas 94 

79.'  The  change  from  fresh  to  weathered  granite  and  to  soil 96 

80.  Diagram  to  illustrate  the  chemical  composition  of  a  granite  and  its  residual 

clay 97 

81.  Microphotograph  of  the  soil  from  igneous  rocks  containing  biotite  mica.  . .     98 

82.  Diagram  to  illustrate  the  chemical  composition  of  fresh  and  weathered 

diabase 100 

83.  Soils  mostly  from  mica  schists,  Pennsylvania 101 

84.  Diagram  of  the  Blue  Grass  region  of  Kentucky  to  illustrate  soil  inheritance  103 

85.  Two  diagrams  of  the  Tishomingo  formation  to  show  inherited  soils  resulting 

from  erosion 104 

86.  Diagram  of  the  junction  of  the  Coastal  Plain  and  Piedmont  Plateau 105 

87.  Diagram  to  illustrate  inherited  soils 105 

88.  A  stratum  of  white  volcanic  dust  (pumicite)  9  feet  thick  lies  between 

strata  of  clay  about  the  middle  of  the  hill.  The  pumicite  is  volcanic 
dust  and  is  believed  to  have  been  transported  hundreds  of  miles  by 
the  winds 108 

89.  Profile  of  a  dune.     The  arrows  show  wind  directions 109 

90.  A  dune  advancing  on  a  forest,  Indiana 109 

91.  Tree  planting  to  hold  "Creeping  Joe,"  a  traveling  dune,  Michigan 110 

92.  A  wind-abraded  rock  surface,  Arizona Ill 

93.  "Blowing"  of  soil  due  to  the  destruction  of  protective  vegetation,  Mich- 

igan   Ill 

94.  Columnar  appearance  of  loess,  Louisiana 112 

95.  Steam  shovel  marks  in  loess  about  15  years  old,  Louisiana 113 

96.  Microphotograph  of  loess  particles 113 

97.  The  principal  areas  of  loessial  soils  in  North  America 118 

98.  Loess  areas  in  Louisiana  and  Mississippi 116 

99.  Diagram  to  show  a  common  relation  between  topography  and  ground 

water 121 

100.  Calcareous  tufa,  a  hot  spring  deposit,  California 123 

101.  A  sink  hole,  Tennessee 123 

102.  Microphotograph  of  chert.     Ground  water  has  deposited  the  minute  layers.  124 

103.  The  water  table  in  the  soil  to  the  right  is  depressed  by  coarse  gravel 125 

104.  Soil  and  subsoil  in  loess  and  in  sandy  loam 126 

105.  Diagram  to  illustrate  the  frequent  occurrence  of  hardpan  and  concretions 

between  soil  and  subsoil . .  128 


xiv  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

106.  Soil  concretions 129 

107.  Patches  of  alkali  in  alfalfa,  Arizona 130 

108.  Generalized  diagram  showing  the  catchment  area  east  of  the  Rocky  Moun- 

tains from  which  the  sandstones  carry  the  underground  water  beneath 
the  Plains 131 

109.  Map  showing  the  estimated  number  of  years  required  for  the  land  to  be 

reduced  one  inch  by  erosion 136 

110.  Head  erosion  of  several  streams  producing  an  escarpment,  Texas 137 

111.  Diagram  to  illustrate  the  subhumid  High  Plains,  the  humid  Rolling  Plains 

and  the  dividing  escarpment 138 

112.  Stages  in  the  formation  of  incised  meanders 139 

113.  Diagram  to  illustrate  the  formation  of  an  incised  meander  and  its  asso- 

ciated soils 140 

114.  To  illustrate  the  "slipping  off"  of  an  incised  meander 140 

115.  Destruction  of  the  woodland  without  adequate  reforestation  has  caused 

gullying 141 

116.  Checking  of  soil  erosion  by  brush  dams 142 

117.  Terraces  in  Central  China : 143 

118.  "Bad  Land"  topography,  Nebraska 144 

119.  Fine  river  sediments 148 

120.  Section  of  Missouri  River  deposits  showing  varying  characters  of  the  sed- 

iments    150 

121.  Diagram  to  show  the  downstream  movement  of  an  alluvial  island,  Missouri .  151 

122.  Map  and  profile  of  a  portion  of  the  Mississippi  flood  plain. 152 

123.  The  level  lower  Mississippi  flood  plain  looking  toward  the  river.     Win- 

drowed  sugar  cane  in  the  foreground,  Louisiana 152 

124.  The  "American  Bottoms."     Part  of  the  Mississippi  flood  plain,  Illinois.  . .  153 

125.  Map  of  soil  types  on  a  part  of  the  Mississippi  flood  plain 154 

126.  Soils  on  the  Kansas  River,  Kansas 155 

127.  Soils  of  the  rapid  Sacramento  River,  Cal 156 

128.  A  river  meandering  in  its  flood  plain 157 

129.  Revetment  in  the  Missouri  River  to  prevent  undercutting 157 

130.  Diagram    to    illustrate    the  outward'  and  down-stream   movements  of 

meanders 158 

131.  Former  ox-bow  lakes  shown  by  muck  soils,  Missouri 158 

132.  Partly  filled  ox-bow  lakes,  Louisiana 159 

133.  The  Missouri  River  depositing  sediment  on  the  inside  of  a  meander  at  the 

left 159 

134.  Showing  the  deposition  of  soils  as  a  meander  of  the  Mississippi  moves  down 

stream 160 

135.  Front  lands  and  back  lands,  Kansas 160 

136.  The  alluvial  plain  and  delta  of  the  Mississippi 161 

137.  Low,  loamy  ridges  built  by  former  streams  on  a  flood  plain,  Louisiana 161 

138.  Alluvial  terraces,  Washington 162 

139.  Diagram  to  illustrate  valley  cutting,  valley  filling  and  terrace  making 163 

140.  Diagram  to  illustrate  a  lower,  smoother  young  terrace  and  an  upper,  older 

and  eroded  terracei «,,,,,, 164 


LIST  OF  ILLUSTRATIONS  XV 

FIQ.  PAGE 

141.  Map.    The  Red,  Brazos  and  Colorado  Rivers  rise  in  regions  of  red  permian 

rocks.     The  Trinity  River  rises  in  a  belt  of  chalks  and  marls  which  fur- 
nish calcareous  materials  to  this  river 165 

142.  Map  of  the  "passes"  of  the  lower  Mississippi  delta  showing  the  areas  gained 

by  deposition  and  those  lost  by  wave  and  current  erosion 167 

143.  Soils  of  the  Puyallup  River  delta,  Oregon 167 

144.  Diagram  to  illustrate  possible  stages  in  delta  building 168 

145.  Map  of  the  Mississippi  delta  showing  the  distributaries  from  the  Red  River 

southward 168 

146.  Combined  delta  of  the  Brahmaputra  and  Ganges  Rivers 169 

147.  Seward,  Alaska.     The  town  is  located  on  a  delta  built  by  a  rapid  stream . . .   169 

148.  The  soils  of  the  old  delta  of  the  American  Fork  River,  Utah , . . .   170 

149.  Soils  of  a  part  of  the  Rio  Grande  delta .171 

150.  Two  views  of  a  small  alluvial  fan,  California 172 

151.  Diagram  to  illustrate  the  building  of  an  alluvial  fan 173 

152.  Diagram  to  show  simple  and  coalesced  alluvial  fans 173 

153.  Map  of  northern  part  of  the  valley  of  California 175 

154.  Soils  on  a  portion  of  a  Piedmont  alluvial  fan  at  the  base  of  the  Sierra 

Nevada  Mountains,  California 176 

155.  Alluvial  fans  extending  from  the  base  of  the  Coast  Range,  California . . 177 

156.  Diagram  showing  structure  of  the  Osage  Valley 179 

157.  Diagrams  illustrating  the  cycle  of  erosion 179 

158.  The  rocks  are  weak  and  have  been  worn  to  a  stage  of  early  age,  South- 

western Missouri 180 

159.  Diagram  to  illustrate  the  movements  of  soil  particles  due  to  freezing  and 

thawing  on  level  and  on  steep  slopes 182 

160.  Whiteside  Mountain,  Southern  Appalachians.     Soil  lodging  and  accumu- 

lating on  more  gentle  slopes 182 

161.  A  "shoulder"  of  colluvial  soil  at  the  base  of  a  sandstone  hill 183 

162.  Sheet  wash,  Alaska 183 

163.  Contour  terraces  to  hold  colluvial  soil  and  to  prevent  soil  erosion,  North 

Carolina 184 

164.  Diagram  to  illustrate  the  occurrence  of  residual  and  colluvial  limestone  soils 

on  a  flat-topped  hill  and  a  round-topped  hill 184 

165.  Diagram  to  illustrate  the  effects  of  soil  creep  and  head  erosion  on  limestone 

soils,  Kansas 184 

166.  Diagram  to  illustrate  origin  of  colluvial  materials 185 

167.  Two  views  of  soils  derived  from  sandy  shales.    Where  the  slopes  are  steep 

stony  loams  are  formed,  where  the  slopes  are  gentle  silt  loams  are 
formed 186 

168.  A  sandstone  cliff  with  talus  extending  nearly  to  the  cliff  top 188 

169.  Landslide  and  scar  on  the  mountain  from  which  the  landslide  slipped, 

Colorado. 188 

170.  Map  showing  the  glaciated  portions  of  North  America  and  the  centers  from 

which  the  glaciers  advanced 190 

171.  Map  showing  the  parts  of  Europe  affected  by  continental  glaciation 191 

172.  Mt.  St.  Helens,  Alaska 193 


xvi  LIST  OF  ILLUSTRATIONS 

FIG.  PAGE 

173.  Crevassed  surface  of  a  glacier,  Canada . 193 

174.  Unglaciated  hilltop  above,  Virginia.     Below,  glaciated  hilltop,  Connecticut  195 

175.  Smoothed  and  grooved  glaciated  rock  surfaces 196 

176.  Stone  scratched  and  smoothed  by  glaciers 196 

177.  Diagram  to  illustrate  plucking  by  ice  when  the  rocks  dip  away  from  the 

glaciers  movement 197 

178.  Mt.  Stephens,  with  smoothed  glaciated  lower  slopes  and  rugged  unglaci- 

ated  upper  slopes,  British  Columbia 198 

179.  Debris  accumulated  beneath  a  continental  glacier  in  Greenland 198 

180.  Stoss  side  and  lee  side  of  a  hill  being  abraded  beneath  a  glacier 199 

181.  Long  lines  of  surface  moraines  on  a  glacier,  Alaska 199 

182.  Perched  boulder  of  quartzite  resting  on  marble;  an  erratic 200 

183.  Stone  fences  of  glacial  rocks,  Wisconsin 200 

184.  Glacial  till  lying  on  solid  rock,  New  Jersey 201 

185.  Diagram  to  illustrate  variation  of  drift  thickness  due  to  buried  hills  and 

valleys 202 

186.  Glacial  clay.     Note  the  angular  bits  of  rock  scattered  through  the  elay 

(much  magnified) 202 

187.  Terminal  moraine  on  a  valley  side,  New  York 205 

188.  Map  showing  successive  positions  of  ice  ir  its  retreat 206 

189.  Morainic  soils,  Michigan 206 

190.  Ground  moraine,  Wisconsin 207 

191.  "Side"  view  of  a  drumlin,  Wisconsin 208 

192.  Fan-like  glacial  debris  from  outcrop 209 

193.  Diagram  to  illustrate  some  relations  between  glacial  soils  and  the  under- 

lying rocks  in  Wisconsin 211 

194.  Generalized  diagram  to  illustrate  the  relations  of  rocks  and  soils  when  the 

glacial  movement  was  parallel  to  the  rock  outcrops 211 

195.  Glacial  streams  building  alluvial  fans,  Alaska , 213 

196.  Diagram  showing  glacial  features  in  an  area  in  southeastern  Wisconsin.  . . .   214 

197.  Looking  across  an  outwash  plain  toward  the  terminal  moraine  in  the  back- 

ground, Maine 214 

198.  Sketch  map  of  Long  Island,  N.  Y.,  showing  the  two  terminal  moraines  and 

the  two  outwash  plains 215 

199.  Diagram  to  illustrate  the  relations  of  soils  to  a  terminal  moraine  and  an 

outwash  plain  in  western  Long  Island 216 

200.  Diagram  to  illustrate  the  formation  of  a  terminal  moraine,  a  recessional 

moraine,  outwash  plains,  and  valley  train 217 

201.  An  esker  in  Michigan 218 

202.  Diagrams  to  illustrate  the  smoothing  of  a  rough  preglacial  topography  and 

roughening  of  a  smooth  preglacial  topography 219 

203.  An  area  in  Wisconsin,  showing  probable  preglacial  topography  and  drainage 

and  present  features 219 

204.  Unglaciated  valley,  Utah. 220 

205.  Glaciated  valley,  Utah 220 

206.  Distant  view  and  close  view  of  a  cirque,  Canada 221 

207.  Preglacial  and  postglacial  valleys  of  the  Mississippi  in  southeastern  Iowa.  222 


LIST  OF  ILLUSTRATIONS  xvii 


208.  Section  of  a  marginal  glacial  lake 223 

209.  Diagrams  showing  stages  in  the  glacial  Lake  Maumee. 224 

210.  Some  of  the  shore  lines  of  Lake  Agassiz 225 

211.  Part  of  the  lake  bed  and  shores  of  the  former  Lake  Agassiz  in  North  Dakota  226 

212.  Map  showing  the  greatest  extent  of  the  glacial  Lake  Agassiz.     The  present 

Lake  Winnipeg  occupies  a  small  part  of  the  extinct  lake  bottom 226 

213.  Different  stages  of  the  great  glacial  lakes 227 

214.  A  section  of  drift  in  Illinois 228 

215.  Map  showing  exposures  of  different  glacial  drifts 229 

216.  Map  of  Illinois  showing  different  drifts 230 

217.  Diagram  to  illustrate  the  topography  and  drainage  on  new  drift 231 

218.  Topography  on  Kansan  drift  in  Iowa 232 

219.  lowan  topography  in  Iowa 233 

220.  Map  of  Ohio  showing  land  values  in  dollars  per  acre  in  1909 236 

221.  Morainic  lake  occupying  a  depression  in  a  terminal  moraine,  Montana. . . .  239 

222.  Principal  areas  of  lake  soils  in  United  States 239 

223.  Diagram  to  illustrate  wave  and  current  work 241 

224.  Wave-cut  terrace  and  cliff  of  an  extinct  glacial  lake 242 

225.  Diagram  to  illustrate  the  development  of  shore  currents 242 

226.  Beach  ridge  of  an  extinct  glacial  lake,  Mich 243 

227.  Shore  and  deep-water  soils  of  the  extinct  glacial  Lake  Agassiz 244 

228.  Lacustrine  soils  deposited  in  the  former  Lake  Agassiz  in  northwestern 

Minnesota 244 

229.  One  of  the  series  of  level-topped  deltas  built  one  above  the  other  at  dif- 

ferent lake  levels,  New  York 245 

230.  Soils  of  the  old  delta  which  the  Sheyenne  River  built  into  the  extinct  Lake 

Agassiz 245 

231.  Shore  soils  around  a  portion  of  the  extinct  glacial  Lake  Maumee 246 

232.  The  plain  of  Lake  Agassiz,  North  Dakota 247 

233.  Areas  formerly  covered  by  the  extinct  Lakes  Bonneville  and  Lahontan  in 

the  Great  Basin 249 

234.  View  across  an  arm  of  the  extinct  Lake  Bonneville,  Utah 249 

235.  Glacial  lakes  and  ponds  wholly  or  partly  filled,  North  Dakota 251 

236.  Onions  on  muck  soil.     A  filled  glacial  swamp,  New  York 252 

237.  Map  of  Dismal  Swamp,  Va 252 

238.  Vegetation  filling  a  lake 253 

239.  Diagram  illustrating  the  filling  of  lakes  by  vegetation 253 

240.  The  Everglades  of  Florida— a  map 254 

241.  Section  of  a  part  of  the  Everglades  which  here  occupies  a  shallow  limestone 

basin 255 

242.  Diagram  to  illustrate  the  accumulation  of  peat  and  marl  in  a  filling  lake  or 

swamp. 256 

243.  Pebbles  rounded  by  ocean  waves 259 

244.  Ocean  surf,  Canada 259 

245.  Barrier  beaches  on  the  Texas  coast 260 

246.  Barrier  beaches  and  partly  filled  lagoons  on  Long  Island,  New  York 261 

247.  Reclaimed  tidal  flats,  California 261 


xviii  <  r  LIST  OF  ILLUSTRATIONS 

FIO.  PAGE 

248.  "Sea  Islands,  "South  Carolina 262 

249.  The  submerged  lower  Hudson  Valley 263 

250.  The  submerged  coastal  plain  and  part  of  the  emerged  coastal  plain 264 

251.  A  part  of  the  coastal  plain  in  Alabama 267 

252.  Map  showing  the  general  distribution  of  the  Lafayette  and  Columbia 

formations . 265 

253.  The  coastal  plain,  Va 266 

254.  Ancient  and  extinct  coral,  modern  coral 271 

255.  Coquina  limestone,  coraline  limestone,  massive  limestone 271 

256.  Diagram  showing  a  comparison  of  phosphoric  acid  in  different  rocks 273 

257.  Nodules  of  lime  phosphate 274 

258.  Diagram  showing  the  occurrence  of  brown  phosphate  in  Tennessee 275 

259.  Limestone  "horses"  in  brown  phosphate,  Tennessee 276 

260.  Microphotograph  of  phosphatic  rock 276 

261.  Oolitic  phosphate  rock,  Montana 277 

262.  Diagram  to  illustrate  the    deposition    of   Silurian   phosphatic  waste   in 

Devonian  seas 277 

263.  Map  of  Florida  showing  the  principal  phosphate  areas 278 

264.  Fragment  of  a  boulder  of  rock  phosphate  showing  part  of  a  cavity  lined 

with  crystalline  phosphate  minerals 278 

265.  Phosphatiaed  limestone,  Florida. .......  :\ ',-;'. . : • .;. • .V-.V.  iv;V 279 

236.  An  occurrence  of  phosphate  in  Florida. . . -'. •.  /.:. .' A-i* -. . . :  Av.1  .c/r:?j, 279 

267.  Section  of  the  Stassfurt  salts  beds.. . . . . . . . ........ . . . . .  I.  r.v. . . . .  282 

268.  Soil  map  of  the  United  States .  . ; .' / .\.\ 285 

269.  Diagram  to  illustrate  the  topography  and  structure  of  the  Cumberland 

Plateau,  Appalachian  Valley  and  Ridge  Belt  and  the  Blue  Ridge 
Mountains .'. .-. . •. '-.Vi. 287 

270.  A  cambrian  trilobite V, , ,  .'.V.V.  .*</: .". .  V.'7V.!:. 294 


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AGRICULTURAL  GEOLOGY 


INTRODUCTION 

GEOLOGY  as  a  whole  is  essentially  the  study  of  the  earth's  history. 
All  the  different  lines  of  geological  investigation  contribute  directly 
or  indirectly  to  this  end  and  the  study  of  present-day  processes  helps  to 
explain  what  has  occurred  in  the  past.  To  take  an  example,  Fig.  1 


FIG.  1. — A  rock  record  of  an  ancient  beach.     Note  the  long  rill  marks  made  by 
running  water  and  the  round  spots  (rain  prints)  made  by  falling  rain  drops. 

shows  the  rain  pits  and  rill  marks  as  contained  in  old  sandstone  which 
has  preserved  the  evidences  of  beach  conditions  almost  as  perfectly  as 
may  be  found  after  a  storm  on  a  modern  beach.  By  these  fossil  rain 
prints  and  stream  marks  we  know  that  this  rock  was  accumulated  near 
an  ancient  beach.  The  study  of  present  peat  beds  leads  to  an  under- 


standing  bfknov?  e^arieiettt  ^coal.^beds  were  formed.  The  traces  of 
modern  glaciers  are  similar  to  those  of  very  ancient  glaciers.  Many 
other  instances  might  be  cited  to  show  how  the  observation  of  present 
geological  conditions  enables  us  to  work  out  past  geological  history. 

Divisions. — As  in  other  sciences  different  phases  of  geology  may  be 
considered  or  emphasized  for  special  purposes.  Thus  historical  geology 
treats  primarily  of  the  succession  of  events  as  determined  by  the  study 
of  rocks  and  their  remains  of  plants  and  animals  (fossils) .  Dynamical 
Geology  has  to  do  with  the  forces  which  have  changed  and  are 
now  changing  the  earth.  Physiographic  geology  or  physiography 
deals  with  processes  which  are  now  modifying  the  earth's  surface. 
Structural  geology  is  the  study  of  the  materials  of  the  earth  and  how  they 
are  arranged.  Economic  geology  deals  with  whatever  geological  factors 
may  have  commercial  interest.  Agricultural  geology,  with  which  this 
volume  is  concerned,  deals  mainly  with  soils,  and  to  a  less  extent,  with 
the  origin  of  mineral  fertilizers. 

Fundamental   Ideas 

One  of  the  first  ideas  to  be  acquired  in  the  study  of  geology  is  the 
vast  length  of  time  involved.  The  accumulation  of  an  inch  of  lime- 
stone soils  has  required  a  vastly  longer  period  than  the  length  of  known 
historical  time,  and  a  realization  of  this  should  lead  to  a  conservation  of 
our  soils,  which  have  required  so  long  a  time  for  their  formation  and 
will  require  an  equally  long  time  for  their  replacement.  Even  a  mod- 
erately high  hill  has  usually  been  tens  of  thousands  of  years  in  the 
making  and,  if  a  mountain  range  had  been  started  when  the  Pilgrims 
landed  at  Plymouth  in  1620,  it  is  entirely  probable  that  this  would  not 
be  known  to-day,  so  slow  is  the  process  of  mountain  making. 

Although  the  study  of  soil  origin  is  but  one  of  the  many  viewpoints  of 
geology,  yet  there  is  no  agent  or  process  of  geology  which  is  not  in  some 
way  directly  related  to  soils.  Obviously  when  rocks  weather  or  break 
up  they  form  vast  areas  of  residual  soils  so  that  we  speak  of  granite 
soils,  limestone  soils  and  so  forth,  each  rock  usually  contributing  a  dis- 
tinctive soil.  But  even  the  soil  from  a  rock  like  granite  will  vary  under 
different  conditions,  for  a  granite  soil  in  a  dry  country  differs  notably 
from  one  in  a  humid  region  and  one  in  a  hilly  country  has  different 
features  from  one  in  a  level  region.  Then  soils  from  different  rocks  are 
usually  different  in  composition;  granite  soils  often  have  a  somewhat 
high  potash  content  and  sandstone  soils  usually  have  a  low  content  of 


FUNDAMENTAL  IDEAS  3 

mineral  plant  foods  and,  moreover,  soils  from  different  rocks  often 
differ  both  in  their  soil  minerals  and  in  chemical  composition  so  that 
the  composition  of  the  parent  rocks  must  be  considered. 

About  the  most  stable  thing  that  we  know  is  the  earth's  surface, 
yet  geology  teaches  that  the  oceans  and  the  lands  of  North  America 
and  elsewhere  have  many  times  changed  places  and,  indeed,  are  doing 
so  to-day.  As  a  consequence  of  these  movements  there  have  been 
upward  movements  that  have  brought  large  areas  of  rocks  and  soils,  like 
the  Coastal  Plain  of  North  America,  above  the  ocean  while  downward 
movements  have  submerged  thousands  of  square  miles  of  former  soils. 

Among  the  most  important  soils  are  those  that  have  been  transported 
and  deposited  so  that  some  soils  show  but  little  relation  to  the  under- 
lying rocks.  A  heavy  clay  soil,  for  example,  may  have  been  deposited 
over  sandstone,  thus  giving  a  soil  much  unlike  the  sandy  soil  which 
might  be  expected  from  a  sandstone.  Probably  the  most  widespread 
transporting  agent  is  the  winds  which,  as  we  shall  see,  have  carried  and 
are  now  carrying  vast  amounts  of  soil-forming  materials.  Then,  in  the 
past,  vast  glaciers  overrode  more  than  half  of  North  America  and  con- 
siderable areas  in  Europe.  These  glaciers  had  most  notable  effects  on 
soils  so  that  glacial  soils  are  usually  distinctive,  for  not  only  are  glacial 
soil  materials  more  or  less  mixtures  of  various  materials  that  have  been 
transported,  but  glaciers  further  modified  the  drainage  and  topography, 
both  very  important  soil  and  agricultural  features.  Perhaps  the  most 
familiar  transported  soils  are  those  laid  down  by  running  water,  for 
except  in  very  dry  regions,  there  are  many  streams  and  all  carry  some 
sediment  which  they  deposit  at  various  places.  There  are  large  areas 
of  alluvial  soils  like  those  of  the  Mississippi,  but  even  in  hilly  regions 
with  narrow  valleys,  these  "  bottom  soils  "  are  important  in  value  if  not 
in  area. 

But  the  geological  story  of  soils  is  not  concluded  when  we  have 
considered  their  origin,  for  soils  are  affected  not  only  by  their  origin  and 
materials  but  also  by  the  agents  which  have  modified  them  since  the 
soils  were  formed.  Among  these  are  the  ground  and  soil  water,  which 
here  may  leach  the  soils  of  their  soluble  minerals  and  there  may  deposit 
materials  in  the  soils.  Furthermore  the  movements  of  ground  and  soil 
waters  have  an  obvious  relation  to  soil  moisture  and  to  wells  and  springs. 
Then,  of  course,  soils,  like  rocks,  are  affected  by  weathering,  that  is  by 
temperature  changes,  freezing  and  thawing  and  other  agents.  For 
example,  soils  from  the  same  kind  of  rock  in  North  Carolina  and  New 
Jersey  differ  because  of  the  difference  in  the  agents  and  processes  to 


4  INTRODUCTION 

which  the  soils  have  been  subjected  since  their  formation.  Thus  it  is 
seen  that  all  the  geological  agents  and  processes  combine  in  different 
ways  to  affect  the  soils. 

In  this  study  the  rocks  will  first  be  considered  since  they  are  the 
ultimate  source  of  nearly  all  soils.  We  shall  also  note  the  different 
processes  by  which  soil  materials  are  transported  from  place  to  place  and 
in  general  how  soils  are  affected  by  geological  processes. 


CHAPTER  I 
MINERALS  ! 

AN  observer  by  the  lower  Mississippi  where  the  river  has  deposited 
fine  soils  may  see  no  connection  between  his  soil  and  a  rock,  say,  like 
granite.  But  if  he  examines  the  very  fine  materials  of  soil  through  a 
microscope  he  is  likely  to  find  minerals  which  came  originally  from 
granite-like  rocks.  Hence  it  is  that  we  begin  the  geological  study  of 
soils  with  a  consideration  of  rocks  because  practically  all  soils  except 
muck  must  at  one  time  or  another  have  been  a  part  of  some  rock.  Fur- 
thermore, since  rocks  are  usually  composed  of  two  or  more  minerals, 
the  study  of  minerals  will  be  taken  up  first,  in  order  the  better  to 
understand. the  materials  of  which  rocks  are  made. 

Fortunately  for  simplicity  of  study,  the  common  and  important 
minerals  are  relatively  few,  although  hundreds  have  been  identified. 
Likewise  the  mineral  composition  of  the  earth's  crust  is  comparatively 
simple.  According  to  Clark's  estimate  the  relative  percentages  in  the 
earth's  crust  are  as  follows : 2 

Oxygen 47.33  Titanium. .  *. 46 

Silicon 27.74  Carbon 19 

Aluminum 7 . 85  Phosphorus 12 

Iron 4 . 50  Manganese 08 

Calcium 3 . 47  Sulphur 12 

Magnesium 2 . 24  Barium 08 

Sodium 2.46  Strontium 02 

Potassium 2 . 46  Chlorine 06 

Fluorine .10 

It  will  be  seen  that  the  elements  oxygen  and  silicon  comprise  75 
per  cent  and  these  with  eighteen  other  elements  comprise  nearly  99 
per  cent  of  the  rocks  so  far  as  they  have  been  studied.  Many  important 

1  It  is  hardly  necessary  to  state  that  the  student  must  study  actual  specimens 
in  order  to  gain  a  knowledge  of  minerals. 

2  The  Data  of  Geochemistry,  Bulletin  No.  619,  U.  S.  Geological  Survey,  1916, 
page  34. 

5 


6  MINERALS 

elements,  such  as  lead  and  copper,  show  a  very  small  fraction  of  1  per 
cent  and  do  not  appear  in  the  above  table. 

These  elements  are  nearly  always  combined.  Silicon  and  oxygen 
unite  to  form  the  familiar  mineral  quartz  (SiCfe)  composed  of  one  part  of 
silicon  and  two  parts  of  oxygen.  Pure  iron,  for  example,  is  very  rare 
although  it  is  very  common  in  compounds  the  world  over.  The  number 
of  important  minerals  is  comparatively  small  since  scarcely  more  than  a 
dozen  of  great  groups  include  most  of  the  minerals  to  be  found  in  average 
rocks.  Thus  the  student  may  hope  by  comparatively  brief  study  to 
recognize  most  of  the  minerals  which  he  is  likely  to  find. 

General  Characters  of  Minerals 

Color  is  a  quality  which  is  easily  noted  but  with  many  minerals  the 
color  is  variable.  Pyrite,  "  fools'  gold,"  is  brassy  yellow,  while  calcite, 
the  common  lime  mineral,  may  have  many  colors  although  it  is  usually 
white. 

Luster  is  due  to  reflection  of  light  from  surfaces  of  minerals.  Luster 
may  be  glassy  like  fractured  glass;  resinous  as  in  sphalerite;  pearly  as  in 
mother-of-pearl;  silky  as  with  fibrous  minerals  like  satin  spar  and 
dull  as  in  kaolin.  Many  minerals,  as  for  example,  pyrite,  have  metallic 
luster. 

Streak  is  the  color  of  powdered  mineral  and  with  fairly  soft  minerals 
it  may  be  obtained  by  rubbing  the  mineral  on  a  surface  like  that  of 
unglazed  porcelain.  Harder  minerals  may  be  pulverized.  Streak 
sometimes  varies  from  color,  and  being  fairly  constant  is  a  useful 
characteristic. 

Hardness  of  fresh  minerals  refers  to  the  ease  with  which  they  are 
scratched.  Hardness  is  often  stated  in  terms  of  Mohr's  scale  as  follows, 
the  type  minerals  being  in  order  of  hardness  from  soft  to  hard : 

1.  Talc  6.  Orthoclase 

2.  Crystallized  gypsum  7.  Quartz 

3.  Calcite  8.  Topaz 

4.  Fluorite  9.  Corundum 

5.  Apatite  10.  Diamond 

For  field  determinations  and  for  most  purposes  the  following  scale 
will  be  sufficient: 

Very  soft,  No.  1,  easily  scratched  by  finger  nail. 

Soft,  No.  2,  just  scratched  by  finger  nail. 

Hard,  No.  3,  scratched  by  a  copper  coin;  not  scratched  by  finger 
nail. 


GENERAL  CHARACTERS  OF  MINERALS 


Hard,  No.  4,  not  scratched  by  copper  coin  but  easily  scratched  by  a 
knife. 

Hard,  No.  5,  just  scratches  glass. 

Very  hard,  No.  6,  scratches  glass  easily. 

From  7  to  10  the  hardness  is  difficult  to  determine  and  requires 
considerable  training  for  its  determination,  but  with  a  set  of  standard 
minerals,  one  can  become  used  to  the  "  feel  "  of  hardness. 

Tenacity. — A  mineral  is  brittle  when  easily  broken  to' pieces;  sectile, 
when  it  can  be  shaved  into  fine  slices;  malleable,  when  it  can  be  hammered 
thin ;  elastic,  when  a  thin  portion  is 
bent  it  will  fly  back  to  the  original 
position;  flexible,  when  a  thin  por- 
tion can  be  bent  without  breaking. 

Cleavage.  --  When  minerals 
split  easily  with  smooth  faces  in 
certain  directions  they  are  said  to 
have  the  property  of  cleavage. 
Thus  some  minerals,  like  quartz, 
when  struck  a  blow,  will  break 
into  fragments  of  various  shapes. 

Others,  like  calcite,  break  into  fragments  each  of  the  same  general  shape. 
When  a  mineral  breaks  easily  and  smoothly  along  certain  planes  it  is 

said  to  have  cleavage ;  for  example, 
mica  has  a  good  cleavage  in  one 
direction.  Calcite  has  three  cleav- 
ages which  are  not  at  right  angles. 
Feldspar  cleaves  in  two  directions 


FIG.  2. — Cleavages:  on  the  left  calcite 
at  oblique  angles;  on  the  right,  feld- 
spar at  right  angles  (top  and  bottom). 
The  end  of  the  feldspar  shows  fraction 
instead  of  cleavage. 


nearly  at  right  angles.  (Fig.  2.) 
When  it  is  present,  cleavage  is  a 
very  important  characteristic. 

Fracture  is  a  break  not  along 
smooth  faces  as  in  cleavage.     A 
fracture    somewhat    curved     and 
shell-like  as  the  inside  of  a  shell 
is  termed  conchoidal,  Fig.  3.     This 
fracture  is  beautifully  shown  in 
some  Indian  arrow  heads  shaped 
from  flint  or  chert.     Fibrous  min- 
erals often  show  a  splintery  fracture.     Some  minerals  have  a  jagged 
fracture  like  broken  steel. 


FIG  .  3.— Shell-like  (conchoidal)  fracture 
of  flint.  To  the  right,  an  arrow  head 
of  the  same  material  and  with  the 
same  fracture. 


8  MINERALS 

Crystal  form  is  shown  by  many  minerals.  Quartz,  for  instance, 
often  crystallizes  into  beautiful  six-sided  crystals.  Calcite  is  commonly 

crystallized.     Many  minerals  have 
both  a  crystalline  and  an  uncrys- 
talline   form    and    some   minerals 
have  rarely  or  never  been  found 
crystallized.     The  general  crystal 
form  should,  if  possible,  be  recog- 
FIG.  4.— Crystal   forms;    from   left   to      nized  for  it  is  constant  for  a  given 
right,  apatite,  feldspar,  garnet.  mineral. 

Specific  gravity  is  an  impor- 
tant characteristic  of  minerals.  For  many  purposes,  minerals  may  be 
described  as  follows : 

Specific  gravity  up  to  2,  light;  2  to  4,  medium;  above  4,  heavy. 
Quartz,  having  a  specific  gravity  of  about  2.7,  may  be  remembered  as 
a  common  medium-weight  mineral. 

Important  Soil  and  Rock-making  Minerals 

Apatite  (Ca5(ClF))(PO4)3,  a  phosphate  of  lime,  is  essentially  a  com- 
bination of  lime,  phosphorus  and  oxygen.  The  mineral  is  very  common 
in  many  granite-like  (igneous)  rocks  where  it  may  be  found  in  crystalline 
form  and  the  microscope  shows  that,  in  very  small  crystals,  it  is  a  com- 
mon mineral  of  most  igneous  rocks. 

The  non-crystalline  form  often  occurs  in  limestones,  where  it  is  the 
"  phosphate  "  of  commerce.  In  fact  phosphate  rock  may  be  regarded 
as  an  impure  limestone.  The  phosphorus  in  limestone  is  converted 
into  soluble  form  by  treatment  with  sulphuric  acid.  The  crystalline 
form  of  apatite  has  the  following  characteristics:1  H,  4.5-5;  sp.  gr. 
3.17-3.23;  luster,  vitreous  to  resinous. 

Calcite  (CaCO3)  is  a  combination  of  lime  and  carbonic  acid. 
It  is  a  very  common  mineral  and  is  the  basis  of  limestone.  It 
is  often  crystallized  and  the  crystals  are  commonly  six-sided 
prisms  and  pyramids.  The  cleavage  is  perfect  in  three  directions 
so  as  to  form  rhombohedrons.  The  mineral  is  brittle,  commonly 
white,  but  often  of  various  colors,  including  shades  of  red  and 
brown.  It  is  easily  attacked  by  weak  acids,  forming  carbon  dioxide 
gas,  hence  effervescence  upon  application  of  acid  is  a  rough  test  of 
1  H  indicates  hardness;  sp.  gr.,  specific  gravity. 


IMPORTANT  SOIL  AND  ROCK  MAKING  MINERALS  9 

calcite  and  limestone.  Calcite  is  fairly  soluble  in  water  which  contains 
carbon  dioxide,  thus  forming  the  bicarbonate  which  makes  limewater 
"  hard  "  for  household  use.  Either  in  crystalline  or  amorphous  forms 
it  exists  under  many  names  among  which  are  dog-tooth  spar,  limestone, 
chalk,  calcareous  marl,  onyx,  travertine,  etc.  Aragonite  has  the  same 
composition  as  calcite  but  crystallizes  in  different  forms;  it  is  harder 
and  heavier  and  cleaves  into  prismatic  forms.  The  uses  of  calcite  in 
the  form  of  limestone  are  many;  when  limestone  is  roasted  the  carbon 
dioxide  (C02)  is  driven  off  giving  quicklime  (CaO).  It  is  used  in  glass 
making,  iron  smelting,  and  as  a  corrective  for  soil  sourness.  H,  3; 
sp.  gr.,  2.72. 

Dolomite  (CaMg(C03)2)  is  a  combination  of  calcium,  magnesium 
and  carbon  dioxide.  It  much  resembles  calcite,  from  which  it  is  dis- 
tinguished by  the  fact  that  it  does  not  easily  effervesce  in  cold,  dilute 
acid  as  does  calcite.  Nearly  all  limestones  contain  some  dolomite  but 
the  term  is  usually  restricted  to  those  minerals  having  about  20  per  cent 
or  more  of  magnesium  carbonate  and  the  remainder  of  calcium  car- 
bonate. Limestones  containing  considerable  dolomite  are  termed 
dolomitic  limestones.  They  are  usually  somewhat  better  for  building 
stone  since  they  do  not  weather  as  readily  as  the  pure  limestones.  Both 
calcite  and  dolomite  are  water-deposited  minerals  and  are  very  frequent 
in  many  mineral  veins  where  water  has  been  an  important  depositing 
agent. 

Gypsum  (CaS04  •  2H2<3)  is  a  sulphate  of  lime  combined  with  water. 
It  occurs  in  crystalline  form  (selenite)  or  in  a  compact  fine-grained  form 
(alabaster)  or  as  granular,  earthy  and  often  impure  form  known  as 
rock  gypsum.  The  crystalline  form  is  a  soft,  clear  mineral  which  can 
easily  be  split  into  thin,  somewhat  flexible  plates.  Most  of  the  gypsum 
of  commerce  is  obtained  from  the  rock  gypsum  which,  however,  nearly 
always  contains  small  crystals  of  selenite.  When  gypsum  is  heated 
in  a  closed  tube  it  readily  gives  off  its  combined  water.  Gypsum  is 
sparingly  soluble  in  water  and  the  principal  deposits  have  been  made  by 
the  evaporation  of  enclosed  bodies  of  water.  It  is  extensively  ground 
and  applied  to  the  soil  as  "  land  plaster."  When  heated  so  as  to  drive 
off  the  combined  water,  gypsum  becomes  the  "  plaster-of-Paris  "  of 
commerce.  Gypsum  in  crystalline  form  (selenite)  has  the  following 
characteristics:  H,  2;  sp.  gr.  2.31-2.33. 

Halite,  rock  salt,  "  salt  "  (NaCl)  is  a  combination  of  sodium  and 
chlorine.  It  is  a  widely  distributed  and  very  soluble  mineral,  the 
qualities  of  which  are  so  familiar  as  not  to  require  description.  Many 


10  MINERALS 

coarse  deposits  are  colored  reddish  shades  by  compounds  of  iron 
H,  2.5;  sp.  gr.  2.4-2.6. 

Nitre  ("  saltpeter  ")  (KNO3)  is  formed  by  the  action  of  nitric  acid 
on  compounds  of  potassium.  It  is  a  white,  easily  soluble  mineral  some- 
times seen  in  thin  crusts  on  walls  and  on  decomposing  animal  and  vege- 
table matter.  The  solubility  of  nitre  makes  it  a  quickly  available 
potash  fertilizer  but  its  principal  use  is  in  the  manufacture  of  explosives. 
.  Kainite  (MgSCU,  KC1,  3H20)  is  a  soluble  mineral  of  variable  com- 
position. It  is  widely  used  as  a  potash  fertilizer.  The  main  supply  is 
from  Germany,  where  the  kainite  is  associated  with  salt  and  other  sol- 
uble minerals.  The  colors  vary  from  white  to  red. 

Trona,  "black  alkali"  (Na2CO3  •  NaHC03 .  2H2O)  is  a  glistening, 
whitish,  soluble,  bitter  mineral,  known  in  our  arid  and  sub-arid  regions  as 
"  black  alkali."  It  is  an  injurious  mineral  in  soils;  it  puddles  clays,  is 
injurious  to  plants  and  it  dissolves  humus,  forming  a  black  solution 
which,  upon  evaporating  forms  black  spots,  hence  the  name. 

Mirabilite>  "  glauber  salt"  (Na2SO4-10H2O),  a  soft,  white  light 
mineral  (sp.  gr.  1.5)  which  forms  most  of  the  "  white  alkali  "  of  the  arid 
regions.  It  is  not  a  desirable  ingredient  in  soils  but  is  not  so  injurious  as 
"  black  alkali."  Both  of  these  undesirable  minerals  are  practically  con- 
fined to  soils  in  dry  regions.  They  are  brought  near  the  surface  by  the 
action  of  ground  water. 

Iron  Minerals 

Iron  is  only  rarely  found  in  the  native  state  but  in  combination  it 
is  very  widespread.  The  principal  iron  minerals  are  as  follows: 

Hematite  (Fe2Os),  an  oxide  of  iron,  occurs  mainly  in  amorphous 
masses  with  reddish  to  reddish-brown  colors  and  it  always  has  a  red 
streak.  In  rocks  and  especially  in  soils  and  subsoils  it  is  widely  dis- 
tributed and  is  one  of  the  causes  of  their  reddish  colors.  H,  5.5-6.5; 
sp.  gr.  4.5-5.3. 

Limonite  (2Fe2Os-3H20)  is  an  extremely  common  mineral.  One  of 
its  common  occurrences  is  "  iron  rust."  It  is  yellowish  to  brownish  in 
color  sometimes  compact  but  of  ten  earthy,  when  it  is  known  as  "  ochre." 
Limonite  is  distinctively  an  alteration  mineral,  that  is,  it  has  been 
changed  from  some  other  mineral  often  by  the  addition  of  water.  For 
example,  one  way  by  which  limonite  may  be  formed  is  by  the  addition 
of  water  to  hematite  according  to  the  following  equation : 

Hematite     added  to    Water     yields        Limonite 
2Fe2O3  +  3H2O         =        2Fe2O3-3H2O. 


SILICA  AND  SILICATES  11 

Limonite  is  the  chief  cause  of  the  yellowish-colored  tints  of  rocks  and 
soils;  its  presence  is  an  important  factor  in  differentiating  soils,  not  so 
much  because  of  itself,  but,  as  we  shall  see  later,  as  an  indicator  of 
aeration  and  water  circulation.  It  is  an  abundant  ore  but  often  impure. 
H,  5-5.5;  sp.  gr.  3.6-4  (both  for  the  compact  form  only). 

Magnetite  (FesCU)  is  also  an  oxide  of  iron  usually  with  metallic  luster 
and  black  streak.  It  is  often  crystallized  in  cube-like  forms  and  its 
common  name,  "  loadstone,"  as  well  as  its  specific  name  suggest  its 
prominent  characteristic;  it  is  strongly  attracted  by  a  magnet  and  is 
an  important  but  not  widespread  iron  ore.  It  is  found  in  many  crystal- 
line rocks  in  microscopic  crystals  and  so  abundant  is  the  magnetite  in 
some  boulders  that  they  will  deflect  a  compass  needle.  H,  5.5-6.5; 
sp.  gr.  4.9-5.2. 

Siderite  (FeCOs),  is  a  carbonate  of  iron  with  colors  varying  through 
gray,  brown  to  black.  It  is  an  important  ore  locally.  H,  3.5  -4;  sp.  gr. 
3.83. 

Pyrite  (FeS2),  is  a  sulphide  of  iron.  The  brassy  yellow  color  has 
given  the  mineral  its  common  name,  "  fools'  gold."  Pyrite  is  used  only 
for  its  sulphur  which  is  obtained  by  roasting.  Its  presence  in  iron  ore 
in  any  but  very  small  percentages  renders  the  ore  useless  for  most 
purposes.  Pyrite  is  common  in  many  rocks,  where  it  is  rather  easily 
altered,  an  example  being  as  follows: 

Pyrite  added  to  Oxygen   and    Water    yields      Limonite       and  Oxide  of  sulphur 
4FeS2         4-          22.0        +      3H2O        =     2Fe2O3-3H2O      +          8SO2 

The  oxide  of  sulphur  thus  produced  changes  to  sulphuric  acid 
(H2SO4),  which  vigorously  attacks  the  rocks  and  breaks  them  down. 
Pyrite  causes  the  "  sulphur  smell  "  in  some  coal  smoke.  H,  6-6.5; 
sp.  gr.  4.9-5.2;  streak,  greenish  black  in  contrast  to  the  yellowish 
color. 

Silica  and  the  Silicates 

Silica  (Si02)  occurs  in  quartz,  the  crystalline  form,  and  in  amorphous 
forms  such  as  flint,  chert  and  chalcedony,  which  are  non-crystalline. 
Quartz  is  hard  and  brittle,  usually  somewhat  transparent  and  has  a 
glassy  luster.  When  crystallized,  it  shows  six-sided  forms,  both  prisms 
and  pyramids.  Some  crystalline  quartz  with  a  certain  purple  color  is 
the  gem  amethyst.  Chalcedony  and  agate  are  non-crystalline  forms. 
Flint  and  chert  are  more  or  less  impure  varieties  and  usually  have  a 


12  MINERALS 

conchoidal  fracture,  a  feature  that,  together  with  their  hardness,  made 
them  valuable  for  primitive  weapons. 

Quartz  is  common  the  world  over  because  it  is  a  very  stable  and 
resistant  mineral  and  our  common  sands  are,  therefore,  predominantly 
of  quartz.  It  is  used  as  a  filler  for  mortar  and  concrete,  as  an  abrasive, 
in  the  manufacture  of  glass  and  has  many  other  uses.  Quartz  is  an 
important  constituent  of  many  rocks  and  is  a  very  important  constit- 
uent of  most  soils.  >  Silica  in  various  forms  is  extensively  deposited 
by  water  in  its  slow  underground  movements.  This  is  possible 
because  silica  is  slightly  soluble  in  alkaline  solutions.  H,  7;  sp. 
gr.  2.7. 

The  silicates  are  extremely  important  rock-forming  minerals.  The 
name  is  applied  from  the  fact  that  these  minerals  are  various  com- 
pounds of  silica  (8162) .  In  the  silicates  the  silica  acts  as  an  acid  radical 
in  combination  with  a  base.  To  take  a  familiar  example,  if  hydro- 
chloric acid  is  applied  to  lime  we  obtain  a  new  compound  of  lime  and 
chlorine,  CaC^.  In  somewhat  the  same  way  we  have,  for  example,  a 
silicate,  wollastonite,  CaSiOs,  composed  of  calcium  as  the  base  and  silica 
as  the  acid.  A  large  and  important  group  of  silicates  contain  aluminum 
and  are,  therefore,  termed  aluminum  silicates. 

The  feldspars  are  silicates  of  aluminum  with  bases  of  potash,  soda 
and  lime.  The  feldspars  are  hard,  of  medium  weight  and  have  two 
cleavages  about  at  right  angles.  They  are  very  important  and  wide- 
spread minerals  in  certain  classes  of  rocks. 

Orthoclase,  the  potash  feldspar  (KAlSisOs),  is  of  various  colors  rang- 
ing mostly  from  white  to  reddish  tints.  It  is  an  essential  mineral  of 
granite  and  an  important  mineral  in  many  other  rocks.  As  a  glaze  for 
china  and  earthenware  it  has  an  extensive  use.  It  is  an  extremely 
important  soil  mineral  not  only  in  soils  from  granite-like  rocks  in  which 
it  is  a  common  mineral  but  also  in  many  clay  and  silt  soils  in  which  it  has 
been  deposited.  Orthoclase  breaks  down  fairly  easily  in  part  because 
of  its  good  cleavage  which  makes  easier  the  work  of  ice  and  other  agents. 
In  decomposing,  orthoclase  yields  sand,  clay  and  some  compound  of 
potash.  Soils  which  contain  orthoclase  have  small  bits  scattered 
through  them  and  these  small  particles  are  continually  giving  off  soluble 
compounds  of  potash.  Orthoclase  has  to  a  limited  extent  been  ground 
and  used  as  a  fertilizer  since  it  contains  something  like  17  per  cent  of 
potash.  The  mineral  is  regarded  as  the  original  source  of  most  com- 
mercial potash.  H,  6-6.5;  sp.  gr.  2.45-2.62;  streak  white;  luster 
vitreous  to  pearly. 


SILICA  AND  SILICATES  13 

The  plagioclase  feldspars  include  albite,  the  soda  feldspar, 
(NaAlSisOg),  oligoclase,  the  soda  lime  feldspar,  labradorite,  the  lime 
soda  feldspar  and  anorthite,  the  lime  feldspar  (CaAl2Si20g). 

It  will  be  seen  that  oligoclase  and  labradorite  are  mixtures  of  albite 
and  anorthite.  These  feldspars  are  somewhat  alike  in  appearance  and 
their  determination  is  often  somewhat  difficult.  One  feature  rather 
common  to  the  plagioclases  is  the  fine  striations  often  seen  on  the  smooth 
cleavage  faces.  These  feldspars  decompose  rather  readily,  yielding 
sand,  clay  and  compounds  of  lime  and  soda. 

Micas  are  characterized  by  the  well-known  perfect  one-direction 
cleavage.  Muscovite,  white  mica,  commonly  known  as  "  isinglass,"  is  a 
potash  mica  (H2(KNa)Al3(SiC>4)3).  Biotite  is  the  black  mica  sometimes 
termed  the  iron-magnesia  mica  ((HK)2(MgFe)2Al2(Si04)3).  While  the 
micas  cleave  very  readily  and  so  are  easily  broken  into  fine  fragments, 
they  do  not  readily  decompose.  As  a  consequence  the  mineral  is  wide- 
spread even  in  rocks  that  are  not  its  primary  source  and  a  close  examina- 
tion of  almost  any  handful  of  fine  sand  will  reveal  bits  of  mica.  Since 
micas  do  not  readily  decompose  they  are  not  important  soil  minerals 
from  the  chemical  viewpoint,  but  on  the  other  hand,  the  fine  flakes 
scattered  through  a  soil  render  it  more  open  textured.  When  mica  is 
scattered  through  a  rock  its  relative  weakness  aids  in  the  breaking  up  of 
the  rock  into  soils.  In  some  rocks  mica  is  so  abundant  locally  that  a 
considerable  proportion  of  the  stream  "  sand "  is  of  this  mineral. 
H,  2-2.5;  sp.  gr.,  2.5-3;  tenacity,  elastic. 

Olivine  ((MgFe)  28164)  is  a  glassy-appearing,  brittle  mineral  of  vari- 
ous colors.  While  not  common  in  large  masses  it  is  widely  distributed 
through  many  rocks.  It  changes  readily  to  other  minerals  and  thereby 
aids  in  breaking  down  a  rock  into  soil.  H,  6.5-7;  sp.  gr.,  3.2-3.5. 

Hornblende  and  augite  are  important  rock-making  minerals.  They 
are  much  the  same  in  composition  but  have  different  crystal  forms. 
Both  are  shining  black  to  greenish-black  hard  minerals  and  in  composi- 
tion are  complex  and  somewhat  variable  silicates  of  lime,  magnesium, 
manganese,  iron,  soda  and  potash.  Augite,  when  well  crystallized,  is 
usually  somewhat  thicker  and  shorter  than  the  hornblende  crystals. 
Hornblende  crystals  are  usually  long  and  of  needle-like  appearance. 
These  minerals  are  commonly  somewhat  massive  in  rocks  and  often 
cannot  be  readily  differentiated. 

Both  hornblende  and  augite  decompose  rather .  easily,  the  former 
more  easily  than  the  latter  because  of  its  easier  cleavage.  Many 
"  speckled  granites  "  have  hornblende  as  an  important  mineral.  The 


14  MINERALS 

rusty  stains  on  some  granites  are  due  largely  to  iron  oxides  derived 
from  decomposed  hornblende.     Hornblende  has  H,  5-6;  sp.  gr.  2.9-3.4. 

Secondary  silicates  are  derived  from  other  silicates,  when  other  com- 
pounds, especially  water,  are  added  to  form  new  minerals.  For  example, 
olivine  may,  by  the  addition  of  oxygen  and  water,  change  to  serpentine,  a 
waxy-appearing  mineral.  The  zeolites,  usually  light-colored  minerals, 
are  hydrous  silicates  with  much  combined  water.  The  water  is  so 
weakly  combined  that  slight  heating  will  drive  it  off,  hence  the  name 
from  a  Greek  word  meaning  to  boil,  so  called  because  these  minerals 
swell  and  lose  their  water  with  an  appearance  of  boiling  when  heated 
The  following  are  among  the  zeolites  of  agricultural  importance: 
apophyllite,  a  potash  zeolite,  Hi4K2Ca8(Si03)io-9H2O;  stilbite, 
H4(Na2Ca)Al2(SiO3)G-4H2O;  analcite,  NaAl(Si03)2-H20. 

Zeolites  are  not  widespread,  although  they  are  found  abundantly 
locally.  They  occur  typically  in  the  crevices  and  cavities  of  some  rocks. 
It  is  probable  that  they  are  somewhat  common  in  many  soils  although 
their  presence  has  not  been  definitely  proved.  They  are  rather  unstable 
minerals  and  easily  break  down.  It  is  a  well-known  fact  that  when  a 
soluble  salt,  say  soda,  is  introduced  into  a  soil,  the  solution  from  the 
soil  may  contain  potash  instead  of  soda,  evidently  an  exchange  of  bases. 
Hilgard  and  others  hold  that  this  exchange  of  bases  takes  place  between 
soil  zeolites.  Furthermore  zeolites  are  readily  soluble  and  their  bases 
are  readily  available  for  plants. 

Talc  (H^Mga (SiOa).*),  is  a  soft,  light-colored  mineral  with  a  charac- 
teristic "  soapy  "  feel.  It  is  derived  from  magnesian  silicates.  Ser- 
pentine (H4Mg3Si2Oo)  is  also  a  secondary  mineral  derived  from  silicates 
rich  in  magnesia.  It  is  of  variable  colors  with  a  somewhat  waxy  luster. 
Soils  derived  from  serpentines  are  likely  to  be  infertile  and  such  area 
are  sometimes  called  "  serpentine  barrens. " 

Glauconite,  "  greensand,"  is  a  hydrated  aluminum  silicate  of  iron 
and  potassium  containing  also  lime,  magnesia  and  soda.  It  is  usually 
in  greenish  grains,  hence  the  common  name  "  greensand."  It  is  found 
in  many  formations  but  is  especially  abundant  in  the  Cretaceous  of  the 
Coastal  Plain.  In  the  United  States  the  most  notable  locality  is  in 
southern  New  Jersey.  Glauconite  has  been  used  to  some  extent  as  a 
fertilizer  on  account  of  its  potash  and  lime. 

Kaolinite  has  the  formula  H4Al2Si2Oo.  It  is  an  extremely  important 
and  widespread  mineral  and  is  the  principal  mineral  of  many  clays  which 
are  so  important  a  constituent  of  soils.  Kaolin  is  entirely  an  alteration 
mineral  resulting  from  the  decomposition  of  silicates,  especially  the 


SILICA  AND  SILICATES  15 

feldspars.  When  a  feldspar,  for  example,  breaks  down,  there  results 
kaolinite,  quartz  and  some  compound  of  soda,  potash  or  lime.  The 
finer  grades  of  porcelain  are  made  from  kaolin.  Ordinarily,  kaolin  is 
light  in  color,  somewhat  crumbly  and  of  light  weight. 

Ferro  magnesian  minerals,  as  the  name  implies,  are  those  minerals, 
especially  silicates,  which  are  high  in  iron  and  lime.  Such  are  horn- 
blende, augite,  biotite  and  olivine.  It  will  be  convenient  to  use  this 
term  frequently  in  later  chapters. 

REFERENCES 

E.  S.  DANA,  Minerals  and  How  to  Study  Them,  Wiley,  1915. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  2nd 

Edition,  Chapters  1-3. 

L.  V.  PIRSSON,  Rocks  and  Rock  Minerals,  Wiley,  1908. 
RIES  AND  WATSON,  Engineering  Geology,  Wiley  &  Sons,  1914,  Chapter  1 


CHAPTER  II 
ROCKS 

Mantle  Rock. — Except  in  small  and  relatively  rare  areas,  the  earth's 
surface  with  which  we  are  familiar  is  composed  of  loose,  unconsolidated 
materials  called  the  mantle  rock,  because  it  overlies  the  underlying  or 
bed  rock  that  is  to  be  found  the  world  over  beneath  this  mantle  rock. 
The  upper  part  of  this  mantle  rock,  the  part  that  supports  plant  growth, 
is  the  soil. 

In  ordinary  usage  the  term  rock  implies  a  certain  solidity  and  hard- 
ness, but  in  geological  usage  the  term  is  broader  and  includes  incoherent 
masses  such  as  sand  and  clay  as  well  as  granites  and  other  solid,  hard 
rocks.  It  is  mainly  from  rocks  that  the  earth's  history  has  been  deci- 
phered. 

Classification. — It  is  evident  that  many  different  criteria  might  be 
used  in  the  classification  of  rocks.  They  might  be  grouped  according 
to  origin  or  chemical  composition  or  texture,  or  according  to  the  pre- 
dominant minerals;  in  fact  all  the  above-mentioned  criteria  are  used  in 
the  different  classifications. 

With  respect  to  origin  there  are  three  great  divisions.  Igneous 
rocks  have  been  cooled  from  a  molten  condition.  Sedimentary  rocks 
have  been  carried  and  deposited  by  wind,  water  or  ice;  the  term  is 
applied  irrespective  as  to  whether  the  sediments  are  loose  or  consolidated. 
Metamorphic  rocks  have  been  greatly  changed,  either  chemically  or 
physically.  So  far  as  their  chemical  composition  is  concerned,  rocks 
may  be  divided  into  two  classes.  Those  with  a  high  percentage  of  silica 
(SiO2),  65  per  cent  or  more,  are  termed  acidic;  those  with  high  per- 
centages of  iron,  calcium,  magnesium  or  sodium  are  termed  basic; 
basic  rocks  contain  50  per  cent  or  less  of  silica.  From  the  point  of 
view  of  their  physical  composition,  rocks  may  be  classified  according  to 
their  texture — whether  the  particles  are  coarse  or  fine-grained  or  whether 
they  are  easily  seen  or  practically  invisible. 

16 


IGNEOUS  ROCKS  17 

IGNEOUS  ROCKS 

Igneous  rocks  have  been  cooled  from  a  former  molten  condition, 
an  origin  often  revealed  by  features  associated  with  flowing  lava.  Thus 
flowage  lines  which  were  developed  by  unequal  flowage  in  the  original 
lava  are  frequently  reserved,  and  beds  of  porous  lava  and  volcanic  ash 
are  sometimes  found  in  ancient  igneous  rocks.  One  convenient  classifi- 
cation is  that  by  which  igneous  rocks  are  distinguished  by  the  depth 
below  the  earth's  surface  at  which  they  were  cooled;  deep  seated  igneous 
rocks  are  termed  plutonic,  while  those  formed  at  or  near  the  surface  as 
volcanic  lavas  are  termed  extrusive.  But  while  these  rocks  differ  in 
some  respects,  it  should  be  remembered  that  they  grade  into  each  other 
and  in  geology  as  in  other  sciences  there  are  few,  if  any,  sharply  dividing 
lines.  Another  useful  term  is  magma,  by  which  is  meant  the  former 
molten  condition  of  igneous  rocks.  The  term  is  closely  synonymous 
with  the  term  lava,  but  the  latter  term  is  usually  restricted  to  modern 
surface  flows.  Obviously  the  volcanic  lavas  recently  erupted  are 
easily  studied;  on  the  other  hand,  erosion  has  not  only  worn  down  vol- 
canoes so  their  interior  structures  can  be  studied,  but  it  has  exposed  to 
observation  deep-lying  plutonic  masses,  Fig.  22.  Igneous  rocks  are 
considered  to  be  the  primary  rocks  from  which  all  others  have  been 
derived,  either  directly  or  indirectly,  and  they,  therefore,  naturally 
come  first  in  the  study  of  rocks. 

Composition. — While  the  number  of  elements  in  most  igneous  rocks 
is  small,  the  number  of  possible  combinations  is  large.  However,  the 
prominent  and  important  minerals  are  relatively  few.  The  following 
table  is  an  estimate,  based  on  a  great  many  analyses,  of  the  average 
mineralogical  composition  of  igneous  rocks  i1 

Feldspar 59.5%      Biotite 3.8% 

Minerals  of  the  hornblende  and  augite  class .  .   16.8          Titanium  minerals . .  1.5 
Quartz 12.00          Apatite 0.6 

The  large  amounts  of  apatite  and  feldspar  are  of  agricultural  interest 
since  the  igneous  rocks  are  probably  the  principal  original  source  of  our 
phosphorus,  potash,  lime  and  soda.  It  will  be  seen  that  the  minerals 
of  igneous  rocks  are  largely  silicates,  which  are  combinations  of  silica 
(Si02)  with  various  bases.  For  example,  if  the  magma  of  an  igneous 
rock  is  high  in  potassium  it  may  contain  considerable  orthoclase  feld- 
spar; if  high  in  magnesium  and  iron,  olivine  is  likely  to  be  formed;  if 
1  F.  W.  Clark,  Bulletin  330,  U.  S.  Geological  Survey. 


18 


ROCKS 


low  in  silica  but  high  in  iron,  magnesium  and  calcium,  the  predominant 
minerals  are  likely  to  be  ferro-magnesian.     When  the  magma  is  high  in 

silica,  some  of  the  silica  cannot  come  into 
combination  since  there  are  not  enough 
basic  materials  and  free  quartz  will  there- 
fore separate  and  crystallize  by  itself. 

Texture  of  an  igneous  rock  depends 
primarily  on  the  rate  of  cooling.  The 
solution  of  salt  in  water  is  an  analogy;  if 
this  solution  is  allowed  to  evaporate  slowly, 
large,  distinct  grains  may  be  formed,  but  if 
the  evaporation  is  rapid,  the  result  is  a 
more  or  less  amorphous  mass  of  salt. 
Similarly,  if  a  magma  cools  slowly  under 
favorable  circumstances,  the  elements 
assemble  into  minerals  which  crystallize 
and  form  a  mass  mainly  composed  of  inter- 
locking crystals  either  large  or  small.  Such 
a  texture  is  termed  a  granitoid  texture,  of 
which  granite  is  a  well-known  example,  Figs.  5  and  6.  If,  on  the  other 


FIG.  5. — Granite  showing  gra- 
nitoid texture.  (Daly,  Ca- 
nadian Geological  Survey.) 


FIG.  6.— Granitoid  texture.  A  microphotograph  of  a  thin  section  of  igneous  rock, 
highly  magnified.  The  light-colored  minerals  are  mostly  feldspars.  (U.  S. 
Geological  Survey.) 


IGNEOUS  ROCKS 


19 


FIG.  7. — Glassy  textures  of  obsidian. 


hand,  the  cooling  is  rapid,  there  are  no  definite  minerals  crystallized  and 
the  resulting  rock  is  amorphous  and  glassy.  Such  is  termed  a  glassy 
texture,  Fig.  7.  Glass  and  furnace  slag  are  examples  of  glassy  textures. 
Under  certain  conditions  some  of  the  minerals  will  crystallize  into  definite 
crystals  termed  phenocrysts 
and  the  remainder  of  the 
magma  will  remain  glassy  or 
composed  of  very  fine  crystals; 
the  latter  portion  is  termed 
the  ground  mass.  Such  a 
texture  is  termed  porphyritic, 
Fig.  8. 

While  the  rate  of  cooling 
is  the  primary  factor  in  the 
production  of  texture,  other 
factors  have  a  more  or  less 
indirect  influence.  (1)  Pres- 
sure and  dissolved  vapors  such 

as  steam  have  some  effect.  (2)  Temperature  obviously  is  important 
for,  in  general,  the  higher  the  original  temperature  the  more  time 
is  consumed  in  cooling  with  a  resulting  tendency  towards  a  coarser 
texture.  (3)  The  composition  of  the  magma  is  important  since  the 

melting  varies  with  the  composi- 
tion. The  most  acid  or  siliceous 
magmas  are  least  fusible  and  the 
more  basic  are  more  fusible.  Hence 
it  is  that  highly  siliceous  rocks  will 
more  quickly  chill,  and  other  things 
being  equal,  tend  towards  glassy  or 
porphyritic  textures  while  the  basic 
rocks  chill  less  easily  and  tend 
towards  the  porphyritic  and  grani- 
toid textures.  For  this  reason  basic 
lavas  tend  to  flow  farther  from  the 
vent  than  acid  lavas.  (4)  It  is 
evident  that  deeply  buried  plutonic 
magmas  will  produce  rocks  with  coarser  texture  than  extrusive  rocks. 
Very  commonly  an  old  lava  flow  will  show  glassy  texture  at  the 
margins,  porphyritic  texture  for  a  distance  within  and  at  the  center 
will  be  found  granitoid  textures,  the  difference  being  due  to  different 


FIG.  8.— Porphyritic  texture.  The  light- 
colored  phenocrysts  of  feldspar  are 
imbedded  in  a  dark-colored  mass. 


20  ROCKS 

)  v, 

rates  of  cooling.  Finally  it  should  be  remembered  that  there  are  all 
gradations  between  textures  because  there  are  all  gradations  between 
the  factors  producing  textures. 

Classification  of  igneous  rocks  is  made  on  a  threefold  basis  according 
to  texture,  minerals  and  chemical  composition.  Any  magma,  acid  or 
basic,  may  have  any  texture,  although  as  we  have  seen  some  magmas 
favor  certain  textures  according  to  circumstances.  Two  magmas  may 
have  the  same  chemical  composition  but  different  minerals,  as,  for 
example,  one  rock  may  contain  hornblende  and  the  other  augite,  both 
minerals  having  substantially  the  same  composition  but  different  forms. 
It  is  convenient  to  designate  certain  minerals  as  essential  minerals  when 
they  are  characteristic  of  a  certain  rock  while  all  others  are  termed 
accessory  minerals.  By  the  use  of  the  microscope  in  rock  determina- 
tions a  large  number  of  minerals  have  been  described  and  named,  but 
the  close  distinctions  made  possible  by  this  method  constitutes  the  work 
of  the  trained  geologist  and  is  of  little  use  to  the  average  field  worker. 
For  ordinary  purposes,  a  method  of  classification  in  considerable  use  is 
based  on  easily  recognized  features  such  as  color,  texture  and  a  small 
number  of  easily  distinguished  minerals.  Sharp  distinctions  are  often 
not  possible  and  in  some  cases  identifications  must  be  made  by  more 
elaborate  methods. 

DESCRIPTIONS  OF  IGNEOUS  ROCKS 

Granitoid  Texture. — The  igneous  rocks  with  granitoid  texture  and 
minerals  that  for  the  most  part  can  be  distinguished  with  the  unaided 
eye  include  granite,  syenite,  diorite  and  gabbro. 

Granite  has  a  granitoid  texture  with  quartz  and  feldspar  as  the 
essential  minerals.  Mica  is  usually  present  and  mica  and  hornblende 
are  the  most  common  accessory  minerals.  Granites  are  formed  from  a 
somewhat  siliceous  magma  since  there  must  be  an  excess  of  silica  which 
crystallizes  as  quartz.  When  accessory  minerals  are  in  considerable 
quantity,  the  name  of  the  principal  accessory  minerals  is  placed  as  a 
prefix  to  the  rock  name,  as  for  example,  hornblende  granite  or  musco- 
vite-biotite  granite,  etc.  Apatite  is  nearly  always  present  but  in  small 
quantities  and  usually  in  grains, of  microscopic  size.  Granites  with  no 
accessory  minerals  are  termed  binary  granites. 

In  general,  granites  are  light-colored  rocks,  the  colors  being  largely 
due  to  the  feldspars  since  the  quartz  is  generally  colorless.  Reddish 
feldspars  afford  the  pink  and  red  granites  of  commerce.  A  prepon- 


IGNEOUS  ROCKS 


21 


derance  of  muscovite  with  light-colored  feldspars  gives  gray  or  white 
granite.  There  is  naturally  a  wide  variation  in  the  texture  of  granites 
ranging  from  fine  to  very  coarse  grains.  A  pegmatite  is  a  coarse-grained 
granite  which  sometimes  yields  large  sheets  of  mica  and  is  the  main 
source  of  orthoclase. 


•25* 


/    50% 


SILICA,     61    02          70.91;? 
ALUMINA,    Al20a  18.  \*% 
IRON  OXIDES             1.61JS 
LIME,    C8  0               2.92# 
80DA,    NJ20               1.33JS 
POTASH,    K20          6.33* 

| 
1 

75* 


FIG.  9. — Diagram  showing  the  chemical  composition  of  a  biotite  granite.     (After 
Daly,  Canadian  Geological  Survey.) 


Granite  is  the  best-known  igneous  rock  because  of  its  wide  use  as 
a  building  stone.  Its  ease  of  quarrying  and  working,  attractive  colors, 
durability  and  high  crushing  strength  make  it  desirable  for  construction 
purposes.  Fine-grained  varieties  are  used  for  statuary.  It  should  be 
remembered  that,  somewhat  unfortunately,  the  term  granite  is  com- 
monly applied  to  all  igneous  rocks  that  are  used  commercially. 


QUARTZ  34. 3 

ORTHOCLASE  FELDSPAR  37. 

PLAGIOCLASE  25. 1 
BIOTITE    MICA  2. 

MAGNETITE  1. 


son 


FIG.  10. — Mineralogical  composition  of  a  biotite  granite. 

Geological  Survey.) 


(After  Daly,  Canadian 


Fig.  9  shows  the  chemical  composition  of  a  biotite  granite  and  Fig.  10 
the  principal  minerals.  It  will  be  seen  that  silica  comprises  nearly 
three-fourths  of  the  entire  rock  and  yet  only  about  one-third  is  free 
quartz.  It  will  be  noted,  however,  that  most  of  the  minerals  are  silicates 
so  that  the  silica  is  combined  and  not  free  as  quartz.  The  potash  com- 
bines with  the  alumina  and  silica  to  form  orthoclase  and  the  lime  and 
soda,  in  like  manner,  combine  to  form  the  plagioclase  feldspars.  The 
biotite  mica  results  from  a  union  of  iron  and  other  bases  with  alumina 
and  silica.  It  will  be  noted  that,  for  some  unknown  reason,  all  the  iron 


22 


ROCKS 


did  not  combine  but  a  small  amount  separated  to  form  the  oxide  magne- 
tite.    The  rock  is  light  gray  in  color. 

Syenite  consists  of  feldspar  with  little  or  no  quartz  and  usually 
small  amounts  of  hornblende  or  mica.  It  resembles  granite  in  general 
appearance  and  often  a  somewhat  careful  determination  is  required  to 
determine  the  absence  of  quartz,  especially  if  the  rock  is  finely  crystal- 


SILICA  54.06$l 

ALUMINA  18.  75$| 
IRON  OXIDES    7.74$ 

MAGNESIA  2.75$ 

LIME  7.35$ 

SODA  4. 60$ 

POTASH  3. 00$ 

FIG.  11. — Diagram  showing  the  chemical  composition  of  a  syenite. 
Canadian  Geological  Survey.) 


(After  Daly, 


lized.     It  is  not   an   important   rock   so   far   as   surface   exposure   is 
concerned. 

Figs.  11  and  12  show  the  chemical  and  mineralogical  composition  of 
a  basic  syenite,  that  is,  a  syenite  with  high  content  of  iron  and  other  bases. 
Practically  all  the  silica  has  combined  with  alumina  and  bases  to  form 
silicates  and  less  than  1  per  cent  has  crystallized  as  quartz.  Many 
syenites  have  a  higher  percentage  of  quartz  and  grade  into  the  granites. 


FELDSPAR  63. 5# 
HORNBLENDE  22.8$ 
AUGITE  9.0j£ 

MAGNETITE  1.8$ 
APATITE  1.3$ 

QUARTZ  0.4$ 


25% 


50$ 


FIG.  12.- 


-Diagram  showing  the  mineralogical  composition  of  a  syenite,  the  chemical 
composition  of  which  is  shown  in  Fig.  11. 


A  marked  contrast  with  granite  is  the  high  feldspar  content.  The 
ferro-magnesian  minerals,  hornblende  and  augite,  are  due  to  the  high 
percentage  of  iron  and  other  bases  and  it  will  be  noted  that  here,  again, 
some  iron  remained  uncombined  and  crystallized  as  the  magnetic  iron 
oxide,  magnetite.  The  potash  combines  to  form  orthoclase  feldspar. 
This  rock  shows  a  high  content  of  apatite.  From  the  predominance  of 
the  accessory  ferro-magnesian  minerals,  the  rock  is  terrned  a  horn- 
blende-augite  syenite. 


IGNEOUS  ROCKS 


23 


Diorite. — This  is  a  rather  common  dark-colored  rock.  It  consists 
essentially  of  hornblende  and  feldspar,  and  it  often  contains  minor  quan- 
tities of  quartz  and  biotite.  It  is  a  rock  somewhat  intermediate  in 
composition  between  the  acid  and  the  basic  rocks.  While  the  texture 
is  granitoid,  yet  the  mineral  grains  are  usually  small  and  the  rock  fine 
textured  and  compact,  so  that  it  is  frequently  difficult  to  identify  a 


25% 


50% 


SILICA  56.9$ 

ALUMINA  18.17$ 

IRON  OXIDES  7.11$ 

MAGNESIA  4.36$ 

LIME  6.5 

SODA  3.23$ 

POTASH  1.57$ 

FIG.  13. — Chemical  composition  of  a  diorite.     (After  Daly,  Canadian  Geological 

Survey.) 

diorite  without  a  microscopical  examination.  They  are  often  called 
"  greenstones  "  because  of  their  frequent  greenish-black  color.  Like 
many  other  dark  siliceous  rocks,  diorite  is  sometimes  called  "  trap  rock." 
Figs.  13  and  14  show  the  compositions  of  a  diorite.  As  compared 
with  the  granites,  the  silica  and  potash  have  decreased,  and  with  the 
practical  absence  of  potash,  the  orthoclase  has  disappeared.  The 
higher  lime  and  soda  account  for  the  predominance  of  labradorite.  The 

0  25%  50% 


QUARTZ  9.5$ 

LABRADORITE  58.8$ 
HORNBLENDE  12.8$ 
AUGITE  4.3$ 

BIOTITE  12.5$ 

MAGNETITE  .8$ 


FIG.  14.- 


-Mineralogical  composition  of  a  diorite,  the  chemical  composition  of  which 
is  shown  in  Fig.  13.     (After  Daly,  Canadian  Geological  Survey.) 


high  iron  and  other  bases  combine  to  form  the  ferro-magnesian  minerals 
hornblende,  augite  and  biotite.  The  reason  for  the  small  amount  of 
quartz  as  compared  with  granite  is  that  the  magma  had  a  small  silica 
content  and  hence  there  was  a  smaller  amount  of  uncombined  silica  to 
separate  as  free  quartz.  The  rock  is  usually  dark  brown  to  greenish- 
gray  in  color. 

Gabbro. — This  rock  is  usually  dark  in  color,  coarse    grained  and 
heavy.     Gabbros  are  derived  from  magmas  that  are  relatively  poor  in 


24 


ROCKS 


silica  and  potassium  while  richer  in  iron,  magnesia  and  lime.  From 
this  composition  it  will  be  seen  that  the  predominating  feldspars  would 
be  the  lime  feldspars,  labradorite  and  anorthite.  Magnetite  is  usually 
present  and  gabbro  boulders  sometimes  contain  so  much  magnetite  as 
:to  deflect  a  delicate  magnetic  needle.  Apatite  and  olivine  are  common 
while  quartz  is  naturally  low  in  quantity.  Their  granitoid  texture 


25J5 


50% 


SILICA  47. 

ALUMINA  18. 

IRON  OXIDES  11. 

MAGNESIA  4.  1 

LIME  9.39 

SODA  3.61$ 

POTASB  0.47$ 


FIG.  15. — Chemical  composition  of  a  gabbro.     (After  Daly,  Canadian  Geological 

Survey.) 

indicates  slow  cooling  deep  in  the  earth's  crust  and  therefore  the 
gabbros  are  distinctively  plutonic  rocks.  They  are  easily  recognized 
but  not  widely  distributed  rocks. 

Figs.  15  and  16  show  the  co/npositions  of  a  gabbro.  The  silica  is 
low  and  is  mostly  combined.  The  high  lime  and  soda  account  for  the 
labradorite,  which  constitutes  over  half  of  the  rock.  The  high  per- 
centage of  hornblende  and  augite  is  explained  by  the  abundance  of  iron 

0  25#  50* 


LABRADORITE  57.5$ 
HORNBLENDE  21.8$ 
AUGITE  12.0$ 

BIOTITE  3. 0$ 

MAGNETITE  3.6$ 
APATITE  1.6$ 

QUARTZ  0.5$ 

FIG.   16. — Mineralogical  composition  of  the  gabbro,  the  chemical  composition  of 
which  is  shown  in  Fig.  15.     (After  Daly,  Canadian  Geological  Survey). 

and  magnesia.     The  rock  is  termed  a  hornblende-augite  gabbro  and  is 
grayish  in  color  and  heavy. 

Porphyritic  Texture. — It  will  be  remembered  that,  under  some  con- 
ditions, magmas  cool  so  that  some  minerals  are  able  to  crystallize 
while  other  minerals  are  unable  to  crystallize.  A  rock  formed  under 
such  circumstances  is  termed  a  porphyry  and  the  distinguishing  prefix 
to  the  name  is  that  of  the  dominant  phenocryst.  For  example,  if  most 


IGNEOUS  ROCKS  25 

of  the  phenocrysts  are  quartz  the  rock  is  a  quartz  porphyry;  if  of  feld- 
spar, the  rock  is  a  feldspar  porphyry,  Fig.  8.  Porphyries  usually  have  a 
mottled  appearance  since  the  ground  mass  and  the  phenocrysts  are 
ordinarily  of  different  colors. 

Felsitic  Texture. — Some  igneous  rocks  have  a  crystalline  texture,  but 
the  crystals  are  too  small  to  be  visible.  Such  a  texture  is  intermediate 
between  the  granitoid  and  the  glassy  textures  and  is  termed  felsitic  tex- 
ture. The  light-colored  rocks  with  this  texture  are  called  felsites,  the 
dark-colored  rocks  are  called  basalts. 

Felsites  are  stony-appearing  rocks  of  moderate  weight.  Grays, 
yellows  and  reds  are  common  colors.  They  are  usually  hard  rocks  and 
break  with  a  conchoidal  fracture.  Certain  felsites  called  rhyolites 
have  a  wide  distribution  and  often  show  flowage  lines  and  other  features 
common  to  lava  flows.  Most  felsites  are  of  acid  or  intermediate  com- 
position. 

Basalts  are  heavy,  dark-colored,  dense  and  somewhat  basic  rocks  of 
felsitic  texture.  In  composition  they  are  similar  to  the  gabbros.  They 
are  derived  from  somewhat  basic  magmas  which  fused  easily  and  flowed 
readily.  Basalts  are  very  common  volcanic  rocks  and  are  now  ejected 
by  many  modern  volcanoes.  The  vast  lava  flows  in  Oregon,  Idaho  and 
Washington  (Fig.  28)  are  mostly  composed  of  basalt.  Basalts  often 
show  cavities  due  to  expanding  gases  and  these  cavities  sometimes  have 
minerals  deposited  in  them.  Some  of  the  copper  deposits  of  Michigan 
consist  of  copper  in  such  cavities.  In  texture  the  basalts  are  typically 
fine  grained,  although  there  is  considerable  variation.  A  deep  basaltic 
lava  flow,  for  example,  may  be  felsitic  in  the  upper  part  and  somewhat 
porphyritic  in  its  lower  portions.  The  color  varies  from  gray  to  dark. 
The  term  is  somewhat  inclusive  and  includes  many  dense,  hard,  heavy 
igneous  rocks.  Trap  is  a  term  also  applied  to  this  rock. 

Glassy  Rocks. — These  rocks  are  formed  by  a  quick  chilling  of  their 
magmas  so  that  practicality  no  minerals  crystallize.  In  general,  rocks 
of  this  texture  are  likely  to  be  of  acid  composition,  for  it  will  be  remem- 
bered that  siliceous  magmas  fuse  with  difficulty,  and  therefore  chill 
readily.  Moreover,  rocks  with  glassy  texture  are  largely  confined  to 
flows  at  or  near  the  surface  since  here  it  is  that  magmas  are  so  quickly 
cooled  that  glassy  textures  usually  result.  Furnace  slags  are  essen- 
tially basic  glasses  and  often  cannot  easily  be  distinguished  from  vol- 
canic glasses. 

Obsidian  is  the  general  rock  name  applied  to  most  rocks  of  glassy 
texture.  The  colors  range  through  black,  brown  to  red  with  black 


26 


ROCKS 


as  the  most  common  color.  Pitchstone  resembles  obsidian  but  has  a 
somewhat  resinous  or  pitch-like  luster.  Obsidians  are  hard,  usually 
brittle  and  break  with  a  conchoidal  fracture,  Fig.  7.  Pumice  is  a  spongy, 
cellular  lava;  the  cavities  are  due  to  steam  expanding  when  the  rock 
was  erupted.  Pumice  will  float,  often  for  months,  and  has  been 
found  in  deep-sea  dredgings  in  many  different  localities. 

CLASSIFICATION  OF  IGNEOUS  ROCKS  (AFTER  PIRSSON) 
A.  GRANITOID  TEXTURE.     MINERALS  RECOGNIZABLE 


Generally  light-colored  rocks. 


Generally  dark-colored  rocks  high  in  ferro- 
magnesian  minerals. 


Quartz  and  feldspar. 
Granite. 

Feldspar  with  little 
quartz. 

Syenite. 

Ferro  -  magnesian 
minerals       and 
feldspars. 
Diorite. 

Ferro  -  magnesian 
minerals      with 
small     feldspar 
content. 
Gabbro. 

B.  PORPHYRITIC  TEXTURE.     MINERALS  MOSTLY  RECOGNIZABLE 

Porphyries 


C.  FELSITIC    TEXTURE.     FINELY    CRYSTALLINE.     MINERALS    MOSTLY    UNRECOG- 
NIZABLE 


Light-colored  rocks. 

Felsite. 


Dark-colored  rocks. 
Basalt. 


D.  GLASSY  TEXTURE 
Obsidian 


REFERENCES 

R.  A.  DALY,  Igneous  Rocks  and  Their  Origin,  McGraw-Hill,  1914. 

J.  F.  KEMP,  Handbook  of  Rocks,  Van  Nostrand,  1911,  Chapters  2-6.     The  Igneous 

Rocks. 
GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  Chapter 

on  Igneous  Rocks. 


OCCURRENCES  OF  IGNEOUS  ROCKS  27 


OCCURRENCES  OF  IGNEOUS  ROCKS 

At  this  point  it  will  be  of  interest  to  consider  some  of  the  forms  of 
igneous  rocks  and  their  modes  of  occurrence,  for  the  rocks  on  the  one 
hand  and  their  various  occurrences  on  the  other  hand  help  to  explain  each 
other.  We  have  noted  the  division  of  igneous  rocks  into  two  classes, 
the  extrusive,  in  which  lavas  are  poured  out  on  the  earth's  surface  or  on 
sea  bottoms,  and  the  intrusive,  in  which  magmas  (see  page  17),  are  forced 
into  rocks  beneath  the  surface.  Since  vulcanism  is  considered  in  a  fol- 
lowing chapter,  it  will  be  sufficient  at  this  point  to  state  that  extrusive 
rocks  are  due  to  volcanic  agencies  and  defer  discussion  to  following  pages. 

INTRUSIVE  FORMS. — By  far  the  largest  areas  of  known  igneous 
rocks  have  not  been  extruded  as  volcanic  lavas,  but  have  been  forced 
into  rock  far  beneath  the  earth's  surface  and  are  now  exposed  only  by 
the  erosion  of  formerly  overlying  rocks.  It  is  convenient  to  recognize 
different  forms  and  shapes  although  they  often  grade  into  each  other 
and,  furthermore,  portions  exposed  to  observation  are  often  too  small 
for  the  form  or  shape  to  be  determined.  Some  of  the  more  common 
forms  are  dikes,  sills,  laccoliths,  bosses,  stocks  and  batholiths. 

Dikes  are  essentially  more  or  less  vertical  fissures  which  have  been 
filled  with  lava  which  has  solidified.  When  they  are  intruded  into  sed- 
imentary rocks,  they  typically  break  across  stratification  planes,  Fig.  18. 
They  range  in  thickness  from  a  fraction  of  an  inch  up  to  hundreds  of 
feet  and  a  length  of  over  one  hundred  miles  has  been  traced.  Dikes  are 
often  interrupted  and  reappear  and  they  commonly  form  branching 
systems  which  are  in  places  very  complex.  In  some  instances  lava 
has  reached  the  surface  and  flowed  out  from 'a  fissure  and  when  the  lava 
cooled  in  the  fissure  a  dike  was  formed ;  such  dikes  often  are  found  in  the 
sides  of  a  volcano.  On  the  other  hand,  many  deep-lying  dikes  can  be 
seen  only  when  the  overlying  rocks  have  been  eroded. 

A  dike  is  often  of  relatively  resistant  rock  and,  therefore,  breaks 
down  less  rapidly  and  makes  somewhat  high  land.  This  is  especially 
the  case  when  dikes  occur  in  sedimentary  rocks.  Moreover,  most 
dikes  are  relatively  thin  and  cool  somewhat  fast  so  that  their  texture  is 
glassy  and  this  texture  is  usually  more  resistant  than  other  textures.  In 
some  cases  dikes  may  extend  as  a  wall-like  mass,  as  seen  in  the  Spanish 
Peaks  region  of  Colorado,  Fig.  17.  However,  a  dike  is  perhaps  more 
often  marked  by  a  range  of  hills  than  by  a  ridge.  Occasionally  a  dike 
is  less  resistant  than  the  adjacent  rock,  especially  if  intruded  into 


28 


ROCKS 


igneous  rock  and  in  such  a  case  the  dike  is  often  marked  by  a  trough-like 
depression.     This  is  often  well  shown  on  some  sea  coasts  where  dikes 


FIG.  17. — The  great  dike,  Spanish  Peaks  Region,  Colo.  The  dike  was  formerly  en- 
closed by  sedimentary  rocks  which  have  been  eroded.  (Hill,  U.  S.  Geological 
Survey.) 

are  eroded  by  the  waves  and  long  fissures  are  formed  which  extend  back 
from  the  shore.     Many  dikes  when  they  occur  in  mountainous  regions 

are  necessarily  of  little  agri- 
cultural interest,  but  in  a  belt 
of  sandstones  and  shales 
(Triassic)  which  extends  from 
New  Jersey  to  North  Carolina, 
the  dikes  are  of  considerable 
importance.  A  somewhat 
typical  occurrence  in  this 
region  is  shown  in  Fig.  18 
where  a  dike  has  been  intruded 
into  shales.  The  dike  is 
marked  by  a  line  of  low  hills, 
one  of  which,  however,  rises 
to  a  considerable  height  as 
"  Round  Top,"  the  hill  made 
famous  in  the  Battle  of  Gettysburg.  The  dike  rock  yields  a  clay  loam 


FIG.  18. — Diagram  of  a  dike  intruded  into 
shales.  The  shales  yield  loams  and  the 
dike,  a  clay  loam.  (Data  from  U.  S.  Bureau 
of  Soils.) 


OCCURRENCES  OF  IGNEOUS  ROCKS 


29 


(Cecil),1  but  in  this  as  in  so  many  other  instances  the  greater  elevation 

of  the  dike  country  promotes 

erosion,  which  tends  to  make 

the     soils     coarse     textured. 

Many  dikes    which    are    not 

topographically  distinguished 

can  be  traced  by  their  derived 

boulders,  often  reddish  in  color, 

which  strew  the  surface. 

Sills  are  somewhat  like 
dikes  except  that  they  are 
more  or  less  horizontal  and 
typically  are  intruded  between 
strata  although  they  may  cut 


FIG.  19. — Diagram  showing  sills  and  dikes. 
The  sill  on  the  right  has  broken  across  a 
stratum  and  branched. 


t 
across  one  stratum  and  be  continued  in  another,  Fig.  19.     Sills  are  best 

developed  in  sedimentary  and  other  rocks  which  are  readily  penetrated 

1 A  soil  series  is  a  group  of  soils  having,  in  general,  the  same  origin  and 
similar  essential  characteristics.  A  soil  type  is  a  minor  division  including  soils  having 
the  same  mechanical  composition.  For  example  the  Cecil  series  are  derived  from 
granites  an^  gneisses  and- are  found  in  the  northern  Piedmont.  The  soils  are  gray  to 
red  in  color  and  are  underlain  by  red  clay  subsoils.  The  soil  type,  Cecil  clay  loam,  is 
one  of  the  soil  types  belonging  to  the  Cecil  series  and  having  the  mechanical  compo- 
sition of  a  clay  loam  as  shown  below. 

Soils  are  divided  according  to  their  mechanical  composition  as  follows,  the  aver- 
age diameter  of  soil  particles  being  given  in  millimeters  (1/25  inch) : 

Sand 1- .  05  mm. 

Silt. .05-. 005  mm. 

Clay below  .  005  mm. 

The  following  soil  classes  are  commonly  recognized.  (Averages  determined 
from  8664  mechanical  analyses;  Bull.  78,  U.  S.  Bureau  of  Soils,  1911,  page  12.)  Sands 
and  sandy  loams  are  called  "  sandy  "  or  "light  soils";  silts  and  silt  loams  are  called 
"  heavy  soils." 


Sands. 

Sand  and  Fine 
Gravel, 
Per  Cent. 

Silt, 
Per  Cent. 

Clay, 
Per  Cent. 

Sandy  loams  

67 

21 

12 

Loams       .           .        .        

44 

40 

16 

Silt  loams 

20 

65 

15 

Clay  loams  

36 

38 

26 

Silty  clay  loams         

14 

61 

25 

Clavs 

22 

36 

42 

30 


ROCKS 


by  the  invading  molten  materials,  and  like  dikes,  they  often  branch  and 
subdivide  in  a  complex  fashion.  Sills  are  often  cut  by  dikes  and  both 
dikes  and  sills  manifestly  grade  into  each  other.  Sills  and  the  adjacent 
strata  are  often  tilted  or  folded  and  thus  give  rise  to  ridges  and  escarp- 
ments of  which  the  well-known 
Palisades  along  the  Hudson 
are  an  example,  where  a  thick 
sill  has  been  uplifted  and 
forms  the  escarpment  shown 
in  Fig.  20.  Sills  and  dikes 
are  common  features  in  a  belt 
of  sand-stones  and  shales 
which,  with  some  interrup- 
tions, extends  from  the  Con- 
necticut Valley  into  North 
Carolina.  When  the  sills  and 
dikes  stand  up  as  ridges,  the 
soils  are  usually  thin  and 
stony  but  when  they  are  well 
worn  down  the  soils  are  typi- 
cally deep,  reddish  and  some- 
what heavy  in  texture. 

Volcanic  necks  are  rem- 
nant cores  of  old  volcanoes. 
They  are  commonly  formed 
as  follows.  A  volcano  is  built 
up  of  fragmentary  materials 
and  lava  and  when  eruptions 
^--^J  cease  the  lava  chills  and 
hardens  in  the  vent  so  as  to 
form  a  kind  of  pillar,  so  to 
speak,  in  the  volcano.  As 
erosion  goes  on  the  weaker 
outer  materials  are  worn  away, 

leaving  the  more  resistant  central  core  projecting.  There  are  all 
stages  from  the  fresh  volcano,  where  the  core  is  not  yet  exposed,  to 
instances  where  the  weaker  materials  around  the  plug  have  been 
worn  away  and,  indeed,  the  plug  itself  has  almost  disappeared. 

Volcanic  plugs  are  found  in  many  localities,  among  them  the  coastal 
plain  of  Texas.     Pilot  Knob,  a  low,  rounded  hill  near  Austin,  Tex., 


FIG.  20. — The  Palisades,  a  sill,  N.  Y.  Distant 
view  above;  close  view,  middle.  (U.  S. 
Geological  Survey.)  The  diagram  below 
shows  the  general  structure  in  the  vicinity. 


OCCURRENCES  OF  IGNEOUS  ROCKS 


31 


Fig.  21,  is  an  example.  Here  was  formerly  an  active  volcano  which  has 
been  so  thoroughly  reduced  by  erosion  that  only  the  filled  throat  remains. 
This  is  composed  of  a  dark  basalt 
which  yields  a  heavy  clay  soil  with 
numerous  boulders. 

Laccolith.  —  When  a  mass  of 
molten  rock  is  so  intruded  beneath 
strata  that  they  are  arched  into  a 
dome-shaped  elevation,the  intrusion 

is  termed  a  laccolith;  it  is  really  a  special  form  of  a  thick  sill.  Sundance 
Mountain,  Fig.  22,  is  an  interesting  example  where  the  arched  overlying 
beds  above  the  intrusion  have  been  entirely  removed  by  erosion  and  the 


FIG.  21. — Pilot  Knob,  Texas,  a  volcanic 
plug.     (After  Hill,  U.  S.  Geological 

Survey.) 


FIG.  22. — Sundance  Mountain,  Wyo.,  a  laccolithic  mountain  (Upper  photo).  The 
sides  of  the  mountain  are  partly  buried  in  waste.  The  diagram  below  shows 
a  mass  of  lava  (black)  which  was  intruded  beneath  overlying  rock  and  arched 
it  upwards.  The  overlying  sedimentary  rocks  have  been  eroded  so  that  the  in- 
truded lava  now  forms  the  mountain.  (After  Darton,  U.  S.  Geological  Survey.) 

intrusive  porphyry  is  seen  resting  on  sedimentary  rocks.     The  lac- 
colith has  been  eroded  to  its  very  roots,  so  to  speak. 

Stocks  or  bosses  are  irregular  masses  of  intruded  igneous  rocks, 
often  with  rudely  circular  or  elliptical  surface  exposures.     They  vary 


32  ROCKS 

from  a  few  hundred  feet  to  several  miles  in  diameter.  Bathyliths  are 
much  the  same  except  that  they  are  much  larger  and  their  surface 
exposures  often  include  thousands  of  square  miles;  they  form  the  core 
of 'many  mountain  ranges.  Probably  these  intrusive  masses  attained 
their  present  positions  by  melting  and  assimilating  the  adjacent  rocks, 
although  naturally  our  knowledge  of  the  method  of  their  intrusion  is 
indefinite.  These  huge  masses  are  usually  exposed  only  by  the  erosion 
of  formerly  overlying  rocks. 

VULCANISM 
;v:  '/:-  ?  ••  ?/'.M:  *  *•(  ;V,    . ;" 

Volcanoes  are  at  once  the  most  spectacular  of  geological  agencies. 
A  volcano  is  not  necessarily  a  mountain,  but  given  a  vent  from  which 
lavas,  cinders  and  other  materials  escape,  a  mound  or  cone  will  ordinarily 
be  built.  Typically,  however,  a  volcano  is  a  conical  hill  or  mountain. 
The  lavas  of  many  modern  volcanoes  are  somewhat  basaltic,  or  in  other 
words  somewhat  basic  in  composition  and  in  cooling  they  often  form  a 
somewhat  porous  basalt.  Naturally,  the  lavas  cool  so  quickly  that 
they  never  assume  the  granitoid  texture,  but  many  volcanic  basalts  are 
porphyritic  and  some  lavas,  even  when  escaping  from  the  crater  con- 
tain crystallized  minerals. 

Ejecta  from  Volcanoes. — From  an  agricultural  viewpoint  volcanoes 
have  considerable  interest  since  there  are  large  areas  of  soils  that  are 
derived  directly  from  lavas.  But  even  more  important  is  the  wide- 
spread volcanic  dust  which  has  been  scattered  by  winds  so  that  these 
fine  materials  undoubtedly  are  incorporated  in  many  soils.  Vulcanism 
treats  primarily  of  the  movements  of  lava  and  associated  materials  and 
for  our  purposes  will  be  considered  under  two  heads,  volcanoes  and 
fissure  flows. 

Three  classes  of  materials  are  ejected  from  volcanoes,  namely, 
lava,  gases,  and  fragments  (pyroclastic  materials).  Lava  is  a  molten 
material  which  may  be  regarded  as  a  solution  of  minerals;  for  example, 
we  might  say  that  a  lava,  which  under  certain  conditions  would  cool 
to  a  granite,  is  a  solution  of  quartz,  feldspar,  mica  and  hornblende,  the 
whole  mass  made  fluid  by  heat. 

As  we  have  seen,  lavas  vary  greatly  in  fluidity,  some  flowing  a  con- 
siderable distance  while  others  become  chilled  and  stiff  soon  after  being 
ejected.  The  lavas  which  flow  for  long  distances  and  cover  large  areas 
are  largely  basic  in  composition,  a  highly  fortunate  circumstance  since 
basic  lavas  in  general  yield  better  soils  than  siliceous  lavas. 


OCCURRENCES  OF  IGNEOUS  ROCKS 


33 


Many  gases  escape  from  volcanoes  of  which  by  far  the  most  important 
is  steam.  It  has  been  estimated  that  a  small  cone  on  Mt.  ^Etna  in 
Sicily  ejected  in  one  hundred 
days  2,100,000  cubic  meters  of 
water  and  this  was  only  a  small 
fraction  of  the  total  water 
ejected  from  this  volcano.  This 
amount  of  water  is  about  the 
estimated  flow  of  the  St.  Law- 
rence River  for  one  second.  In 
eruptions  where  large  quantities 
of  steam  are  ejected,  the  steam 
rises,  is  condensed,  and  mingling 
with  volcanic  dust  and  cinders, 
descends  as  the  dreaded  hot 

mud.  This  mud  when  somewhat  hardened  forms  tuff,  a  sort  of  soft 
shale  which  is  an  important  soil  former  in  places.  Many  volcanoes 
eject  enormous  amounts  of  carbon  dioxide. 

The  solid  matter  ejected  from  volcanoes  consists  of  fragments  of 
hardened  lava  which  are  blown  out  by  the  expansion  of  gases.     In 


FIG.  23. — A  volcanic  bomb. 


FIG.  24. — Castle  Rock,  Nebraska.     Note  the  white  volcanic  dust  at  the  base  of 
the  hill.     (Darton,  U.  S.  Geological  Survey.) 


violent  eruptions  large  blocks  of  hardened  lava  are  thrown  for  great 
distances.  Smaller  fragments,  Fig.  23,  are  variously  termed  according 
to  size;  lapille  are  about  the  size  and  shape  of  nuts  and  bombs  have 
a  roundish  shape  about  the  size  of  an  apple.  Volcanic  sand  and  cinders 


34 


ROCKS 


are  terms  that  are  self-explanatory.    Still  finer  is  volcanic  dust,  which 

will  float  in  the  air  for  days  or  weeks. 

Scoria  and  pumice  are  cellular  and  porous  masses  of  lava;  when  they 

were  ejected  as  lava,  the  confined  steam  expanded  and  produced  their 

characteristic  structure.  All 
these  fragments,  as  one  would 
expect,  are  typically  larger  near 
their  source  and  grade  to  finer 
materials  at  a  distance,  although 
the  grading  is  very  imperfect. 

From  an  agricultural  point  of 
view,  the  finer  dust  is  the  most 
important  of  these  fragments 
because  it  is  carried  so  far  by 
the  winds  and  spread  on  the 
soils.  Widespread  formations  in 
Nebraska  and  adjacent  states 
contain  beds  of  volcanic  dust  the 
origin  of  which  is  unknown,  but 
there  are  no  probable  sources 
within  hundreds  of  miles,  Figs. 
24  and  88.  Every  eruption  where 

dust  is   ejected   furnishes   materials   that   finally  settle  on  soils  and 

become  more  or  less  incorporated  in  them.     It  is  no  exaggeration  to 

state  that  millions  of  acres  have  an  appreciable  amount  of  volcanic 

dust  in   their  soils  and,  indeed,      r 

it  is  probable  that  nearly  every 

square  mile  of  the  earth's  surface 

contains  some  volcanic  dust.     In 

many    cases   volcanic   dust    has 

settled  in  lakes  and  forms  beds 

of    considerable  thickness.     The 

Florissant  beds  of  Colorado  were 

thus  formed  and  in   these   beds 

are  preserved  remarkably  perfect  FIQ   26>_crater  of  an  extinct  volcano  in 

fossil  insects.     A  further  illustra-      Arizona.    (Robinson,   U.   S.   Geological 

tion  is  found  in  lake  beds  located      Survey.) 

in  the  Jefferson  River  Valley  of 

Montana.     Here  heavy  showers  of  white  volcanic  dust  settled  to  the 

bottom  of  lakes.     Upon  this  is  a  stratum  over  1000  feet  in  thick- 


FIG.  25. — Very  fine  volcanic  dust,  Ne- 
braska. Magnified  about  250  diameters. 
It  is  largely  composed  of  angular  bits 
of  glass. 


OCCURRENCES  OF  IGNEOUS  ROCKS 


35 


FIG.  27. — A  recent  lava  flow  in  New  Mexico. 
The  surface  has  a  "ropy"  appearance  due 
to  unequal  flowage.  (Meinzer,  U.  S.  Geo- 
logical Survey.) 


ness  composed  of  reddish  volcanic  dust  which  was  evidently  washed 
into  the  lake  from  the  surrounding  basin.     Furthermore,  there  are, 
in  many  places,  considerable  areas  of  soil-producing  rocks  which  were 
once   volcanic   materials    but 
which    have    become    altered 
by  metamorphism. 

Types  of  Eruptions. — It  is 
convenient  roughly  to  divide 
volcanoes  into  two  types,  ex- 
plosive and  quiet,  according  to 
the  character  of  the  eruptions. 
It  must  be  noted,  however, 
that  not  only  do  these  types 
grade  into  each  other  but  the 
same  volcano  may  at  different 
times  belong  to  the  one  or  the 
other  type.  The  explosive 
type,  as  the  name  implies,  is 
characterized  by  violent  ex- 
pansion of  gases  by  which  fragments  are  sometimes  thrown  to  great 
distances.  Vesuvius  is  perhaps  the  best-known  example  of  this  type, 
but  by  far  the  most  notable  example  is  Krakatoa,  a  volcano  in  the 

East  Indies.  In  1883  this 
volcano  ejected  enormous 
quantities  of  dust  high  into 
the  air  and  this  dust  was 
carried  around  the  world  by 
the  upper  air  currents  and  is 
believed  to  have  been  the 
cause  of  the  extraordinary  red 
sunsets  in  the  autumns  and 
winter  of  1883-84.  Mauna 
Loa  and  Kilauea,  two  vol- 
canoes of  the  Hawaiian 
Islands,  are  examples  of  the 
quiet  type.  The  lava  gradu- 
ally fills  the  craters  and  its  pressure  usually  develops  fissures 
through  which  the  lava  escapes  and  pours  out  in  comparatively 
gentle  floods.  In  the  quiet  type  but  little  fragmentary  material 
is  ejected. 


Jsil/j 
JSlHl 

V- •"Jfd'O  AI\.  ••'••'•  .••»•••/  ^      \ 

^•iS^CSviVv^       *  U^r^^T  — 


FIG.  28. — Map  of  the  Columbia  River  lava 
flows  (dotted  areas.) 


36 


ROCKS 


Fissure  flows,  as  the  name  implies,  are  lava  flows  from  a  fissure 
instead  of  from  a  volcanic  vent.  Here  again,  however,  the  distinction 
is  not  close  since  small  cones  are  usually  scattered  along  a  fissure. 
Indeed  some  geologists  believe  that  some  ancient  flows  were  ejected 
for  the  ^nost  part  from  a  line  of  very  low  volcanoes  instead  of  from 


FIG.  2QA. — View  over  the  Columbia  lava  plateau  in  the  Palouse  district,  Washing- 
ton.   Alfalfa  in  foreground.     (Curtis,  courtesy  of  Professor  Henry  Landes.) 


FIG.  29B. — View  of  the  Grand  Coulee,  a  valley  cut  in  the  Columbia  lava  plateau, 
Washington.     (Curtis,  courtesy  of  Professor  Henry  Landes.) 

fissures.  In  the  great  eruption  of  1783  in  Iceland,  a  flow  of  lava  welled 
from  a  fissure  20  miles  long  with  a  flow  in  places  68  miles  wide.  This 
type  of  eruption  is  rare  at  present,  but  was  very  important  in  earlier 
geological  periods.  In  a  relatively  recent  geological  period  (Tertiary), 
great  floods  of  lava  overspread  an  area  in  Oregon,  Idaho  and  Washing- 
ton estimated  at  over  200,000  miles  in  extent  and  with  an  observed 


CLASTIC   (FRAGMENTAL)  ROCKS  37 

thickness  in  places  of  hundreds  of  feet,  Fig.  28.  The  plateau  of  the 
Deccan  in  India  has  an  even  larger  area  of  lava  ejected  largely  from  fis- 
sures. Such  widely  extended  lava  flows  must  have  been  made  by 
extremely  fluid  lava  which  is  largely  basic,  as  we  would  expect  from  our 
discussion  on  a  previous  page  of  the  relation  between  composition  and 
fluidity.  There  are  but  few  fragmentary  materials  thus  indicating  a 
quiet  type  of  eruption. 

The  lava  flow  in  the  Northwest  was  not  continuous  but  a  succession 
of  flows,  as  is  proved  by  the  buried  soils  and  sediments.  The  lava 
extended  up  valleys  and  around  hills  much  as  a  flood  of  water.  In 
places  some  of  the  flow  features  are  well  preserved  but  on  the  level  areas 
the  easily  decomposed  basalt  has  weathered  to  a  deep,  productive  soil, 
which  under  favorable  moisture  conditions  is  very  fertile. 

REFERENCES 

CHAMBERLIN  AND  SALISBURY,  Geology,  Holt,  1904,  Vol.  1,  Chap.  10. 

B.  K.  EMERSON,  Holyoke  Folio,  U.  S.  Geological  Survey,  1898;  Compare  Soil  Survey 

of  the  Connecticut  Valley,  U.  S.  Bureau  of  Soils,  1903  (sills). 
W.  H.  HOBBS,  Earth  Features  and  Their  Meaning,  Macmillan,  1912,  Chapters  9 

and  10. 

I.  C.  RUSSELL,  Volcanoes  of  North  America,  Macmillan,  New  York,  1897. 
N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Rept.,  Part  1,  U.  S.  Geological 

Survey,  1890-91;  Volcanic  Soils,  pages  239-245. 


CLASTIC   (FRAGMENTAL)   ROCKS 

Clastic  rocks  are  derived  from  the  debris  of  other  rocks  and,  for  this 
reason,  they  are  often  called  fragmented  rocks.  Moreover,  since  they 
are  for  the  most  part  deposited  as  sediments  they  are  often  termed 
sedimentary  rocks.  It  is  a  matter  of  common  observation  that  even  the 
hardest  rocks,  as  granite  for  example,  will  break  up  into  fragments  under 
the  attack  of  the  weather  and  thus  the  materials  for  clastic  rocks  are 
furnished.  The  materials  of  .these  rocks  are  relatively  simple  since  they 
usually  represent  a  final  stage  in  the  decomposition  of  the  parent  rock. 
A  sandstone  derived  from  a  granite,  for  example,  is  largely  composed 
of  the  quartz  that  is  left  after  the  granite  has  thoroughly  decomposed. 

The  Agents 

The  agents  involved  in  the  formation  of  clastic  rocks  are  many  and 
their  effects  will  be  considered  in  greater  detail  in  later  chapters.  By  far 


38  ROCKS 

the  most  important  of  these  rocks  are  those  deposited  by  water.  The 
sand,  clay  and  other  materials  derived  from  the  breaking  up  of  other 
rocks  are  carried  by  streams,  and  for  the  most  part,  they  are  finally 
deposited  in  the  ocean.  The  faster  currents  carry  the  heavier  and 
coarser  fragments  and  the  slower  currents  the  finer  fragments.  The 
waves  and  the  beach  currents  also  sort  the  materials  along  the  shore 
zone.  As  a  result,  sediments  are  often  stratified,  that  is,  they  are 
arranged  more  or  less  in  layers,  the  coarser  and  heavier  having  been 
deposited  by  swift  currents  while  the  finer  sediments  were  deposited  by 
slower  currents.  For  example  a  river  flowing  into  the  sea  would,  in 

flood,  deposit  a  sandy  stratum, 
and  at  low  water,  a  stratum 
of  clay.  Not  only  so  but  the 
waves  and  shore  currents 
might  modify  and  rework  the 
sediments.  Thus  so  many 
clastic  rocks  show  stratifica- 
tion that  this  feature  is  some- 
what distinctive  of  this  class 
of  rocks,  Fig.  30. 

Since  most  sedimentary 
rocks  have  been  deposited  in 
the  ocean,  they  are  said  to  be 

FIG.  30.-Stratified  rocks.  The  projecting  marim  in  Origin'  .  This  com- 
ledges  are  limestone;  the  others  are  shale.  Pels  a  modification  of  the 
(U.  S.  Geological  Survey.)  common  ideas  as  to  the  sta- 

bility of  the  lands  for  much  of 

North  America  has  been  repeatedly  elevated  and  depressed  below  sea 
level.  It  must  also  not  be  forgotten  that  important  formations  have 
been  laid  down  in  lakes  or  even  in  shallow  swamps  and  that  some  rocks 
are  of  wind  origin,  but  the  most  extensive  and  important  sedimentary 
formations  are  of  marine  origin.  Furthermore,  it  is  important  to  note 
from  the  viewpoint  of  soils  that  the  great  soil-forming  formations  are 
mostly  sedimentary  rocks. 

The  study  of  deposits  in  oceans  and  other  waters  therefore  gives 
clews  to  the  formation  of  ancient  sedimentary  rocks.  The  coarser  mate- 
rials, such  as  sand  and  gravel,  are  generally  deposited  near  the  shore, 
for  here  wave  action  is  strongest  and,  moreover,  the  streams  drop  their 
heaviest  load  near  shore.  Weaker  currents  carry  finer  materials  such 
as  fine  sand,  silt  and  clay  out  to  greater  depths  and  more  quiet  water. 


CLASTIC  (FRAGMENTAL  ROCKS)  39 

Hence  it  is  that  one  may  often  trace  a  stratum  or  formation  from  con- 
glomerates and  sandstones  formed  near  shore1  through  shales  which  were 
deposited  in  deeper  water  to  limestones  which  were  accumulated  in  clear 
and  quiet  water  at  a  distance  from  land.  It  must  not  be  assumed,  how- 
ever, that  such  gradations  are  the  rule,  for  there  are  numerous  exceptions 
because  of  changing  conditions  of  deposition. 

Among  other  agents  winds  may  also  carry  and  deposit  fine  materials 
and  their  deposits  often  show  stratification  but  to  less  extent  than  water 
deposits.  Glaciers  have  made  such  vast  surficial  deposits  that  they 
will  be  considered  in  a  later  chapter.  Chemical  precipitates  occur  to  a 
limited  extent  in  the  oceans  and  to  a  considerable  extent  in  closed  bays 
and  lakes,  as,  for  example,  when  the  water  evaporates  from  a  lake  or 
closed  bay,  the  materials  in  solution,  mostly  common  salt,  are  deposited. 
Marine  animals  play  an  important  part  in  the  formation  of  limestones 
and  plants  are  important  factors  in  the  formation  of  peat  and  coal.  The 
principal  classes  of  clastic  rocks  are  sandstones,  conglomerates,  shales 
and  limestones. 

Sandstones  are  more  or  less  indurated  sandy  sediments.  There 
are  sandstones  so  friable  that  they  are  easily  crumbled  with  the  fingers 
and  others,  on  the  other  hand,  are  so  firm  that  they  are  extremely  resist- 
ant and  are  important  mountain  makers.  The  constituent  grains  vary 
in  size  but  are  mostly  rounded  because  their  transportation  by  water 
has  given  them  this  shape.  Furthermore,  the  grains  are  mostly  com- 
posed of  quartz,  because  this  mineral  is  very  resistant  while  the  minerals 
that  may  have  been  associated  with  quartz  in  the  parent  rock  have  been 
worn  away.  The  hardness  of  sandstone  is  mostly  that  of  quartz,  but 
the  coherence  is  mainly  due  to  the  cements;  a  youthful  sandstone  may 
locally  be  cemented  to  a  very  hard  rock  while  an  old  rock  may  be  friable 
if  there  is  little  or  no  cement.  The  cements  are  mainly  siliceous,  cal- 
careous and  ferruginous.  Siliceous  cements  make  hard,  durable  sand- 
stones which  yield  but  slowly  to  weathering  and  erosion.  Such  sand- 
stone makes  many  of  the  ridges  in  the  Appalachian  Ridge  Belt,  Fig.  58. 
Calcareous  cements  are  relatively  weak  and  in  consequence  many  sand- 
stones having  this  cement  break  down  rather  easily  into  deep  soils,  but 
some  calcareous  cements  make  rocks  that  are  easily  worked  and,  there- 
fore, widely  used  as  building  stones.  Ferruginous  cements  usually  give 
firm  sandstones  with  colors  ranging  from  red,  brown  to  bluish.  When 
not  white,  the  colors  of  sandstonse  are  mostly  due  either  to  the  coating 
of  the  grains  or  to  the  cements.  Sandstones  vary  in  weight  and  are 
seldom  as  heavy  as  quartz  because  of  their  pore  spaces.  The  sand 


40 


ROCKS 


grains  do  not  ordinarily  fit  closely  together  so  that  these  rocks  are  usually 
porous  and  often  contain  artesian  well  water. 

Chemical  Composition. — Fig.  31  shows  the  chemical  composition  of 
253  sandstones,  including  many  varieties.  The  high  percentage  of  silica 
is  at  once  evident,  showing  the  high  sand  content.  The  alumina  is 
mainly  in  the  small  amount  of  clay  which  nearly  all  sandstones  carry. 
There  are  considerable  amounts  of  lime  and  potash  which  is  derived 
mainly  from  fragments  of  feldspars.  Part  of  the  iron  and  magnesia 
is  due  to  the  micas  which  are  frequently  found  in  sandstones  and  some 
of  the  lime,  iron  and  silica  represent  the  common  cements. 

While  silica  is  the  essential  mineral  of  sandstone,  clay  is  almost 
always  present,  especially  in  fine-grained  sandstones,  because  the  currents 
which  can  carry  fine  sand  usually  carry  some  clay;  not  infrequently  an 


o 


78.66$ 

4.78$ 


SILICA 

ALUMINA 

IRON  OXIDES  1.38$J| 

LIME  5.52$ 

MAGNESIA  1.17$ 

SODA  0.45$ 

POTASH  1.32.$ 


50  f 


75% 


FIG.  31.— Composite  analysis  of  253  sandstones.      (After  H.  N.  Stokes,  U.  S.  Geo- 
logical Survey.) 


apparently  highly  siliceous  sandstone  will  weather  to  a  loam  instead  of  a 
sandy  loam  because  of  the  clay  in  the  sandstone.  Mica  is  a  common 
constituent,  for  while  mica  is  a  light,  soft  mineral,  it  endures  transpor- 
tation by  currents  and  waves  remarkably  well.  Such  micaceous  sand- 
stones split  easily  since  the  mica  flakes  lie  more  or  less  parallel.  Arkose 
is  a  sandstone  which  contains  notable  amounts  of  feldspar.  The  soils 
from  arkose  are  naturally  somewhat  high  in  lime  and  potash,  other 
things  being  equal. 

Conglomerates  are  cemented  gravels  with  the  same  cements  as  sand- 
stone, Fig.  32.  As  the  rounded  grains  of  sandstone  become  larger  the 
rock  grades  into  a  conglomerate.  Conglomerates  nearly  always  con- 
tain a  considerable  amount  of  sand  but  seldom  much  silt  or  clay  since 
these  materials  are  so  light  that  currents  able  to  carry  gravel  would 
sweep  the  finer  materials  away.  The  gravels  are  usually  of  some  form 
of  silica  such  as  quartz,  flint  or  chert  but  often  there  are  rounded  frag- 
ments of  other  rocks  such  as  granite,  quartzite  or  even  limestone.  Con- 


CLASTIC  (FRAGMENTAL)  ROCKS 


41 


glomerates  are  usually  associated  with  sandstones  since  the  same  con- 
ditions favor  the  formation  of  both.  When  conglomerates  break  down 
into  soils,  they  usually  yield  gravelly  loams.  Breccia  is  much  like  a 
conglomerate  except  that  the  fragments  are  angular  instead  of  rounded. 
Some  breccias  are  not  due  to  stream  action  but  are  cemented  fragments 
that  have  remained  in  place.  Other  breccias  are  water-deposited  but  the 
fragments  have  not  been  carried  far  enough  to  round  them.  There  is 
considerable  pore  space  in  breccias  because  of  the  angularity  of  their 
fragments,  with  the  frequent 
result  that  the  ground  water 
circulates  freely  and  fills  the 
interstices  with  other  minerals. 
Important  zinc  breccias  have 
been  made  in  this  way. 

Shales  in  typical  form  are 
indurated  clays  and  hence  they 
are  often  called  "  mud  stones." 
The  basic  mineral  is  kaolin 
(Al2O3-2SiO2-2H20).  The  term 
is  a  loose  one  since  clays  them- 
selves are  varied  and  the  term 
shale  usually  pertains  more  to 

structure  than  to  composition.  Shales  are  deposited  in  relatively 
quiet  water  and  often  grade  into  sandstones  or  contain  enough  sand 
to  be  termed  sandy  shales.  They  are  usually  arranged  in  layers, 
sometimes  so  thin  that  they  are  called  "  paper  shales."  When  they 
contain  mica,  as  they  frequently  do,  they  split  easily  along  their 
bedding  (stratification)  planes  and  are  often  improperly  called  slates. 
Flagstones  are  sandy  shales  that  split  into  slabs  of  convenient  thick- 
ness for  paving. 

Shales  are  commonly  dark  in  color  but  many  which  contain  some  com- 
pounds of  iron  show  red  to  brown  colors.  Some  black  shales  are  car- 
bonaceous and,  before  the  days  of  abundant  petroleum,  the  distillation 
of  oil  from  such  shales  was  a  considerable  industry.  Shales  are  exten- 
sively used  in  the  manufacture  of  brick,  tile  and  crockery.  They  break 
down  readily  into  soils,  especially  when  the  bedding  planes  are  numerous. 
As  a  rule  the  soils  from  shales  are  heavy  loams,  but  sandy  shales  may 
yield  sandy  loams,  and  there  are  many  gradations  from  clays  and  silt 
loams  to  sandy  loams  in  soils  derived  from  shales.  Such  soils  are 
widespread  and  fairly  productive. 


FIG.  32.— Conglomerate.     (N.  J.  Geologi- 
cal Survey.) 


42 


ROCKS 


Chemical  Composition.  Fig.  33  shows  the  composite  composition 
of  78  shales,  including  all  common  varieties,  and  Fig.  34  shows  for  com- 
parison the  composition  of  kaolin  the  basic  mineral  of  clays.  The 
comparison  of  shales  with  sandstones  shows  a  sharp  decline  in  the 
amount  of  silica  and  an  increase  in  the  alumina.  But  on  comparing 


25% 


50$ 


SILICA  68.38# 

ALUMINA  15.47$ 
IRON  OXIDES    6.49^ 

LIME  8. 12$ 

MAGNESIA  2.45^ 

SODA  1.31$ 

POTASH  3.25$ 


WATER 


6.02' 


FIG.  33. — Composite  analysis  of  78  shales.     (After  Stokes,  U.  S.  Geological  Survey.) 

these  with  corresponding  compounds  in  kaolin  we  note  that  the  shales 
contain  more  silica  and  less  water  and  alumina  than  kaolin.  This 
indicates  that  the  shales  contain  many  impurities.  The  high  silica 
indicates  a  large  amount  of  fine  sand  and  this  sand  content  explains 
the  fact  that  many  apparently  heavy  shales  yield  surprisingly  light  loamy 
soils.  The  alkalies  and  iron  oxides  are  in  part  due  to  undecomposed 
fragments  of  feldspars  and  ferro-magnesian  minerals. 


25% 


50% 


SI  LIC A  46.  50 « 

ALUMINA  39.57 

WATER  13.93$ 


FIG.  34. — Diagram  showing  the  composition  of  pure  kaolinite.     (After  Clarke, 
U.  S.  Geological  Survey.) 


Limestone  is  an  important  and  rather  widely  distributed  rock,  the 
basic  mineral  of  which  is  calcite  (CaCOs).  It  is  widely  used  as  the 
source  of  lime,  which  is  derived  from  limestone  by  roasting  according 
to  the  following  reaction : 


Calcite    yields     quick-lime 
CaCO3         =          CaO 


and    carbon  dioxide 

+  C02 


Limestone  and  other  calcareous  rocks  will  effervesce  upon  the  applica- 
tion of  cold  dilute  acids  and  this  test  offers  an  easy  method  of  identifica- 
tion when  the  observer  is  in  doubt.  There  are  all  gradations  from  soft, 


CLASTIC  (FRAGMENTAL)  ROCKS 


43 


chalky  friable  limestones  to  firm  dense  rock  that  breaks  with  a  concnoidal 
fracture.  The  colors  are  commonly  white  and  gray  but  with  the  addi- 
tion of  carbonaceous  matter  the  color  darkens  and  many  limestones  are 
black;  these,  however,  will  usually  burn  to  white  lime  when  the  car- 
bonaceous matter  is  burned  out.  Less  common  are  limestones  of  brown- 
ish and  reddish  tints  which  are  due  to  varying  amounts  of  iron  oxides. 

By  far  the  most  important  limestones  are  of  combined  organic  and 
marine  origin  as  is  evidenced  by  the  marine  fossils  often  found  in  lime- 
stone, Fig.  35.  On  the  other  hand  there  are  many  limestones  unques- 
tionably formed  in  the  ocean 
that  are  destitute  of  fossils 
because  the  materials  of  lime- 
stone are  soluble  and  may 
be  dissolved  and  redeposited 
many  times  over  and  the 
fossils  thus  obliterated.  Corals 
are  important  limestone 
makers  and  good  illustrations 
are  found  around  modern 
coral  reefs,  where  the  waves 
break  the  coral  skeletons  into 
coral  sand  slight  portions  of 

which  are  dissolved  by  the  sea  water  and  deposited  between  the 
grains,  thus  making  a  coralline  limestone.  Such  limestones,  much 
changed  by  solution  and  redeposition,  but  still  preserving  their  fossil 
coral  structure,  are  found  very  extensively  in  certain  formations.  Not 
only  corals  but  practically  all  the  marine  animals  which  have  calcareous 
skeletons  contribute  to  the  formation  of  limestones.  Mollusks  (two- 
shelled  animals)  and  tiny  one-shelled  creatures  (foraminifera)  have  con- 
tributed their  skeletons  to  limestone  formations.  Coquina,  Fig.  255,  a 
very  recent  limestone,  consists  simply  of  shells  cemented  by  lime  car- 
bonate and  is  doubtless  an  example  of  the  first  stages  in  many  lime- 
stones. 

Very  extensive  beds  of  limestone  originated  as  calcareous  ooze  in 
rather  quiet  water,  either  deep  or  moderately  shallow.  When  such 
deposits  are  accumulated  not  far  from  shores,  they  are  likely  to  contain 
considerable  sediment  as  silt  or  clay  or  sand.  Hence  we  have  the  fre- 
quent gradations  from  limestone  to  shales  or  to  sandy  shales  or  even  to 
sandstones,  Fig.  30.  Non-marine  limestones  are  not  of  wide  extent 
although  they  may  form  thick  masses  locally.  They  may  be  found 


FIG.  35. — A  fossilliferous  limestone.  The 
lowly  animal  forms  have  been  long  ex- 
tinct. (U.  S.  Geological  Survey.) 


44 


ROCKS 


around  many  calcareous  springs,  where  they  sometimes  form  the  attract- 
ive banded  travertine.  Lime  carbonate  may  on  a  small  scale  be  precip- 
itated in  closed  bays  or  even  in  the  open  ocean  but  it  is  doubtful  if  much 
limestone  has  accumulated  by  precipitation. 

Varieties. — The  frequent  gradations  of  limestone  to  shale  and  to 
sandstones  have  been  noted  before.     As  a  matter  of  fact,  practically 

no  limestone  in  nature  is  free 
from  varying  amounts  of  clay 
so  that  limestone  soils  pro- 
duced by  the  decomposition 
of  limestone  are  always  some- 
what clayey.  Chalk  is  a 
crumbly  limestone  composed 
largely  of  microscopic  shells 
and  their  fragments.  Dolo- 
mitic  limestone  is  a  widespread 
limestone  which  contains  con- 
siderable amounts  of  mag- 
nesium carbonate  (MgCOs).  Cherty  or  flinty  limestone  are  also  very 
common  and  give  rise  to  many  types  of  stony  and  loamy  soils.  The 
chert  may  be  fine  grained  and  scattered  through  the  rock  or  it  often 
occurs  in  lenses  or  strata,  Fig.  36.  Some  of  the  chert  was  doubtless 
deposited  with  the  limestone-forming  materials,  but  it  is  believed  that 
much  has  been  deposited  by  the  ground  water  after  the  rock  was  formed. 
Phosphatic  limestones  have  much  agricultural  interest  since  they  contain 
variable  amounts  of  lime  phosphate. 


FIG.  36.— Cherty  limestone.    The  chert  occurs 
in  layers.     (Mo.  Geological  Survey.) 


0                                                25* 

SILICA 

5.19* 

on 

ALUMINA 

.81* 

IRON  OXIDES 

.545? 

LIME 
MAGNESIA 

42.61$ 
7.90* 

mm 

SODA 

.05* 

POTASH 

.33* 

CARBON   DIOXIDE 

FIG.  37. — Composite  analysis  of  345  limestones.      (After  H.  N.  Stokes,  U.  S.  Geo- 
logical Survey.) 

Chemical  Composition. — Fig.  37  shows  the  composite  analysis  of 
345  limestones.  One  at  once  notes  the  high  content  of  lime  and  carbon 
dioxide  which  constitute  the  basic  mineral  calcite  (CaCOs).  The  con- 


CLASTIC  (FRAGMENTAL)  ROCKS  45 

siderable  amount  of  magnesia  represents  the  magnesian  (dolomitic) 
limestones,  which  are  somewhat  widespread.  The  alumina  and  part  of 
the  silica  constitute  the  clay  which  is  found  in  nearly  all  limestones. 
The  iron  content  seems  low  when  it  is  remembered  that  many  limestone 
soils  are  of  reddish  tints,  but  as  we  shall  see  later,  the  color  of  soils  and 
many  fine-grained  rocks  is  largely  conditioned  by  the  fineness  of  grain 
of  the  coloring  materials. 

STRUCTURE  OP  SEDIMENTARY  ROCKS 

An  exposure  of  sandstone  or  shale  will  often  suggest  the  origin. 
Layers  of  sandstone  and  shale  will  often  alternate  much  as  may  be  seen 
in  the  deposits  of  a  roadside  stream.  Such  an  arrangement  of  layers  is 
termed  stratification  and  the  arrangement  is  due  to  the  sorting  action  of 
water  or  sometimes  of  wind  (see  Fig.  30).  Rapidly  moving  water  can 
carry  coarser  materials  than  slow  currents,  a  fact  that  will  be  more  fully 
discussed  later.  If  we  have,  for  example,  a  layer  of  conglomerate 
overlying  a  layer  of  sandstone  which,  in  turn,  is  underlain  by  shale, 
it  can  be  safely  stated  that  the  conglomerate  was  deposited  by  rushing 
water,  the  sandstone  by  swift  currents  and  the  shale  by  relatively 
quiet  water. 

Then  other  characteristics  besides  size  of  materials  may  give  a  strat- 
ified structure;  differences  in  color  or  weight  may  be  marked  in  different 
layers.  Some  sandstones  and  shales  show  bands  of  purple,  red,  dark 
and  white  colors  so  that  local  names  such  as  "  painted  rock  "  are  applied. 
A  single  layer  of  similar  material  is  often  termed  a  stratum  and  very 
thin  layers  are  termed  lamince.  A  group  of  strata  is  often  called  a 
formation.  The  planes  of  division  ^between  strata  are  termed  bedding 
planes. 

Monoclinal  Structure. — Another  structure,  especially  well  shown 
in  some  areas  of  sedimentary  rocks,  is  where  the  rocks  dip  only  in  one 
direction  and  hence  the  structure  is  appropriately  termed  monoclinal 
(one-dip)  structure.  Such  structure  is  to  be  distinguished  from  hori- 
zontal rocks  on  the  one  hand,  and  on  the  other  hand  from  formations 
which  are  bent  so  as  to  dip  in  two  directions  as  shown  in  Fig.  49. 

Over  large  areas  where  there  has  been  no  folding,  the  beds  often  dip 
gently  in  one  direction  and  so  have  monoclinal  structure.  Such  struc- 
ture is  especially  characteristic  of  the  Great  Coastal  Plain  which,  begin- 
ning at  New  York,  sweeps  southward  and  westward  to  Mexico  and 
includes  millions  of  acres.  The  gentle  dips  of  sedimentary  beds  of  the 


46  ROCKS 

Coastal  Plain  is  due  not  to  folding  but  to  their  having  been  laid  down  in 
a  sea  and  later  elevated  and  tilted. 

The  monoclinal  structure  of  a  coastal  plain  is  shown  in  Fig.  251, 
which  represents  a  portion  of  southeastern  Alabama  and,  as  this  will 
serve  as  a  general  illustration  of  this  structure,  it  will  be  considered 
in  some  detail.  First  there  was  an  old  land,  the  Piedmont,  which,  in 
this  locality,  was  to  the  northward.  As  the  Piedmont  was  eroded  the 
resulting  sediments  were  deposited  in  overlying  strata,  the  oldest  at  the 
bottom.  Later  the  ocean  bed  formed  by  these  strata  was  elevated  and 
formed  the  present  Coastal  Plain.  The  formations  dip  to  the  southward 
and,  in  going  from  north  to  south  over  the  Coastal  Plain,  one  would  cross 
successively  younger  beds,  first  the  Cretaceous  and  then  the  younger 
Eocene  outcrops.  Some  strata  are  strong  and  others  weak,  so  that  in 
places  there  are  corresponding  ridges  and  lowlands,  but  in  general  the 
Coastal  Plain,  as  the  name  implies,  is  somewhat  level  or  rolling.  Mono- 
clinal structure  is  shown  on  a  large  scale  in  some  old  rocks.  For  exam- 
ple the  rocks  in  New  York  and  Northern  Pennsylvania  dip  gently 
southward.  The  strata  leading  from  the  eastern  base  of  the  Rocky 
Mountains  dip  away  under  the  Great  Plains  for  hundreds  of  miles  while 
near  the  mountains  they  are  sharply  up  til  ted,  giving  the  ridges  so  well 
shown  in  the  Garden  of  the  Gods.  Some  of  these  strata  carry  water  and 
furnish  the  main  source  of  artesian  water  in  the  Great  Plains,  Fig.  108. 

REFERENCES 

J  F.  KEMP,  Handbook  of  Rocks,  Van  Nostrand,  1911,  Chapters  7-8.  The  Sedi- 
mentary Rocks. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  pages 
99-132.  (Aqueous  Rocks.) 


METAMORPHIC  ROCKS 

These  are  rocks  that  have  been  greatly  changed  from  their  original 
condition.     Igneous  and  sedimentary  rocks  may  be  changed  to  this 
class  or  indeed  one  metamorphic  rock  may  be  changed  into  another. 
The  process  (metamorphism)  may  be  slight  as  when  a  shale  is  hardened 
and  becomes  a  shaly  slate  or,  as  is  often  the  case,  the  metamorphism 
may  be  so  complete  that  one   cannot   tell  from  what  a  metamorphi 
rock  was  derived.     In  general  the  process  of  metamorphism  strengthen 
rather  than  weakens  rocks  but  this  does  not  always  hold ;  limestone  for 
example,  is  usually  a  weaker  rock  than  marble  into  which  limestone  fre 


AGENTS  OF  METAMORPHISM  47 

quently  changes  and  slate  is  almost  always  harder  than  its  parent  rock, 
shale. 

Changes  Produced  by  Metamorphism. — Metamorphism  may  result 
in  changes  that  are  mainly  physical,  as  when  a  pure  limestone  is 
changed  to  a  marble  of  practically  the  same  composition.  Frequently 
there  is  chemical  change,  as  when  orthoclase  feldspar  is  changed  by 
addition  of  water  to  muscovite  mica,  according  to  the  equation  on 
page  49.  Very  often  there  are  mineralogical  changes  by  which  dif- 
ferent minerals  are  developed  from  minerals  already  in  the  rock.  An 
interesting  example  of  this  is  found  in  the  metamorphism  of  impure 
limestones,  which  often  contain  free  quartz,  iron  oxides,  clay,  magnesia 
and,  of  course,  calcite.  Under  metamorphism  these  compounds  often 
unite  to  form  a  great  variety  of  minerals  among  which  are  mica/  feld- 
spars and  garnets. 

Agents  of  Metamorphism 

The  principal  agents  of  metamorphism  are  heat  and  pressure  and  to 
these  are  often  added  the  effects  of  hot  water  and  gases,  especially 
steam.  It  is  obvious  that  such  enormous  temperatures  and  forces 
cannot  be  reproduced  and  studied  in  laboratory  experiments  nor  can 
they  be  studied  in  action.  The  study  of  metamorphism  and  its  agents 
is,  therefore,  particularly  difficult;  much  more  is  known  of  the  effects  of 
metamorphism  than  of  the  agents  that  produce  it. 

Heat,  causing  metamorphism,  may  be  due  to  at  least  three  factors: 
(1)  At  the  depth  of  several  miles  the  weight  of  the  overlying  rocks  is 
enormous.  The  deep-lying  rocks  are  subjected  to  great  compression 
which  produces  heat  for  the  same  reason  that  pounding  a  piece  of  metal 
will  make  it  hot.  (2)  Rocks  in  some  places  have  been  closely  folded  to 
great  depths  and  such  folding,  though  it  was  very  slow,  has  produced 
heat  much  as  the  rapid  bending  of  a  wire  will  heat  it.  (3)  Furthermore 
it  is  obvious  that  highly  heated  lavas  under  some  conditions  will  provide 
heat  for  metamorphism. 

The  influence  of  heat  is  readily  understood  from  common  observa- 
tion. Heat  hardens  and  bakes  rocks,  drives  off  volatile  substances 
and  also  raises  the  temperature  of  water  and  gases  so  that  they  become 
more  effective  agents  of  metamorphism.  Moreover  the  expansion 
and  shrinkage  often  associated  with  heating  and  cooling  break  up  rock 
and  so  facilitate  the  easy  penetration  of  hot  water  and  gases.  Then 
great  heat  tends  to  make  rocks  somewhat  plastic,  a  condition  that  favors 


48 


ROCKS 


metamorphism.  Many  rocks,  especially  shales  near  large  masses  of 
hot  lava,  are  baked  to  a  dense  glassy  substance  termed  hornfels;  clays 
are  baked  to  a  porcelain-like  substance  and  sandstone  to  a  glassy, 

enamel-like  rock.  A  simple 
case  is  seen  when  bituminous 
coal  is  invaded  by  hot  lava 
with  the  result  that  the  coal 
near  the  lava  is  changed  to 
anthracite  or  coke  by  the 
expulsion  of  volatile  sub- 
stances from  the  bituminous 
coal,  Fig.  38, 

Pressure.  —The  great  pres- 
sure caused  by  the  weight  of 
FIG.  38.— Diagram  to  illustrate  the  change     hundreds  of  feet   of  overlying 
from  bituminous  coal  to  anthracite  by  an     rocks   has  been  noted.     Most 
intrusion  of  lava.     (After  Shaler  and  Wood-     of  the  Well-marked   metamor- 
worth,  U.S.  Geological  Survey.)  phigm  due  to    presgure)  how. 

ever,  is  caused  by  the  folding 

of  rocks.  Not  only  does  folding  produce  great  heat  and  pressure  but  it 
tends  to  shatter  the  rocks  and  thus  provide  easy  channels  for  gases  and 
hot  fluids  which  are  in  themselves  important  factors.  An  effect  of 
folding  is  well  illustrated  in 
Pennsylvania,  where  one  may 
follow  a  horizontal  bed  of 
bituminous  coal  to  where  the 
beds  are  folded  and  the  coal 


TRAP  ROCK  > 


COALI 


FIG.  39. — Diagram  to  illustrate  the  meta- 
morphism of  bituminous  coal  to  anthracite 
because  of  folding.  (C  indicates  a  coal  bed.) 


changes  to    anthracite    as    a 

result  of  the  folding,  Fig.  39. 

It  is  largely  for   this   reason 

that  many  metamorphic  rocks  of  economic  value  such  as  marble,  slate 

and   anthracite  are  found  for  the  most   part   in   regions  of   folded 

rocks. 

Gases  and  fluids  are  factors  in  metamorphism  that  produce  great 
complexity.  Water  is  by  far  the  most  abundant  of  all  fluids  and  an 
enormous  amount  is  contained  in  rocks.  Other  gases  such  as  fluorine, 
chlorine  and  carbon  dioxide  are  present  in  some  rocks.  The  water  may 
be  in  chemical  combination  in  minerals  or  in  some  way  not  well  under- 
stood it  may  be  "  occluded  "  or  hidden  in  the  rocks  Rocks  will  melt 
much  more  easily  in  moist  heat  than  in  dry  heat,  some  requiring  only 


AGENTS  OF  METAMORPHISM  49 

about  one-third  the  heat  for  melting  in  moist  heat  as  compared  with 
dry  heat.  Water  is  an  efficient  solvent  and  its  solvent  power  is  greatly 
increased  when  the  water  is  at  the  high  temperatures  encountered  in 
deeply  buried  rocks.  Moreover  such  water  readily  enters  into  various 
combinations.  The  micas,  for  example,  are  hydrated  minerals  very 
common  in  metamorphic  rocks.  One  way  by  which  they  can  be  formed 
is  shown  by  the  following  equation: 

Orthoclase  added  to     water  yields      Muscovite  and  Potassium     and    Quartz 
(potash                                                       (potash  silicate 

feldspar).  mica) 

3KAlSi3O8  +          H2O        =      KH2Al3Si3Oi2  +      K2SiO3         +       5SiO2 


The  potassium  silicate  is  soluble  in  hot  water  and  may  enter  into 
other  combinations.  Another  common  hydrated  mineral  often  found 
in  metamorphic  rocks  is  talc.  Furthermore  circulating  hot  waters  may 
carry  minerals  in  solution  and  redeposit  them  in  the  interstices  of  rocks 
as  in  quartzites  or  it  may 
dissolve  and  recrystallize  the 
minerals  of  a  rock,  a  process 
often  prominent  in  the  forma- 
tion of  marbles. 

Slaty  Cleavage  and  Schist- 
osity.  —  Many  metamorphic 
rocks  are  slightly  banded 
and  split  easily  in  one  or 
more  directions.  Thus  there 
is  slaty  cleavage  in  rocks 
that  are  not  otherwise  much 
changed.  This  cleavage  is 
due  largely  to  the  develop- 
ment of  minute  flakes  of 
mica  along  certain  planes 
whereby  the  rock  splits  more 
easily.  Slaty  cleavage  is 

caused  by  moderate  metamorphism.  When  the  metamorphism  is 
more  vigorous  there  is  a  development  of  many  new  minerals  which  are 
roughly  parallel  with  each  other  and  often  arranged  in  bands,  a  structure 
termed  schistosity,  and  the  rock  is  said  to  be  foliated.  There  is  usually 
easy  splitting  along  these  bands,  Fig.  40, 


FIG.  40. — Foliated  gneiss  produced  by  intense 
folding.     N.  C.     (U.  S.  Geological  Survey.) 


50  ROCKS 

One  way  by  which  slaty  cleavage  and  schistosity  may  be  developed 
is  illustrated  in  Fig.  41.  Suppose,  for  example,  we  have  granite  (A), 

I  the    minerals  of  which  have 

JLl f      no    definite     directions,     and 

that    this   rock    is    subjected 
to    pressure   in  the  direction 
indicated  by  the  arrow.     As 
FIG.  41.— Diagram  to  illustrate  the  develop-     new  minerals  are  formed  they 
ment  of  schistocity  by  pressure.  tend  to  grow  along  directions 

where    the    pressure    is    less, 

that  is,  approximately  at  right  angles  to  the  pressure,  and  so  develop 
slaty  cleavage  or  foliation. 

Complexity  of  Metamorphism. — It  will  be  readily  apparent  that  it 
is  practically  impossible  to  separate  the  effects  of  the  different  agents 
of  metamorphism  in  most  field  studies.  While  for  ease  of  comparison 
they  have  been  considered  separately,  it  should  be  remembered  that  in 
nature  they  act  jointly  and  so  intricately  that  it  is  extremely  difficult 
to  study  their  separate  effects.  Pressure  tends  to  produce  heat,  and 
heat,  by  expanding  the  rocks,  produces  pressure.  Both  heat  and  pres- 
sure facilitate  the  work  of  liquids  and  gases. 

Contact  and  Regional  Metamorphism 

It  is  convenient  to  consider  metamorphism  under  two  phases. 
When  a  mass  of.  molten  material  comes  into  contact  with  other  rocks 
there  are  likely  to  be  changes  at  or  near  the  contact  and  such  meta- 
morphism is  accordingly  termed  contact  or  local  metamorphism.  The 
region  affected  is  relatively  small.  Regional  metamorphism  occurs  on 
a  larger  scale  and  is  due  largely  to  pressure  produced  by  folding. 
Since  this  type  of  pressure  does  not  operate  on  a  small  scale,  the 
metamorphism  affects  large  areas.  Great  areas  in  the  Appalachians 
and  in  the  Rockies  are  underlain  by  rocks  produced  by  regional  met- 
amorphism. It  should  be  kept  in  mind  that  these  two  phases  are  not 
distinct  and  that  each  involves  many  of  the  factors  operating  with  the 
other. 

Contact  metamorphism  naturally  involves  the  intrusive  molten  rock 
and  also  the  rock  into  which  "the  intrusive  lava  is  forced.  The  larger 
the  mass  of  molten  rock  the  greater  will  be  the  metamorphism,  since 
there  is  more  heat  to  be  radiated  from  the  larger  mass.  Furthermore, 
if  the  molten  mass  is  flowing  instead  of  stagnant  there  will  be  greater 


CONTACT  AND  REGIONAL  METAMORPHISM 


51 


heat  effects.  Other  things  being  equal,  deeply  buried  molten  rocks  are 
likely  to  be  especially  effective  since  the  heat  escapes  so  slowly  that  its 
effect  is  prolonged  and,  moreover,  the  associated  hot  gases  and  vapors 
are  in  effect  for  longer  periods. 

An  illustration  of  contact  metamorphism  is  shown  in  Fig.  42.  Here  the  molten 
rock  (diorite)  has  been  forced  through  a  great  thickness  of  conglomerates,  sand- 
stones and  clays.  The  heat  and  vapors  of  the  in- 
truded mass  have  formed  a  zone  of  metamorphosed 
rock  in  places  over  a  mile  wide.  The  metamorphism 
decreases  from  the  central  core  until  the  metamorphic 
rocks  merge  into  the  stratified  sedimentaries.  Fig. 
43  shows  the  metamorphism  of  a  limestone  by  a  large 
n?  -SS  of  lava.  The  same  features  are  illustrated  in 


J ! 


S! 


FIG.  42. 


FIG.  43. 


FIG.  42. — To  illustrate  contact  metamorphism.    (Data  from  U.  S.  Geological  Survey.) 

FIG.  43. — Limestone  (LS)  metamorphosed  by  an  intrusion  of  lava  (GD).  The 
diagram  shows  the  lava  on  the  left  (GD)  which  has  made  a  zone  (M)  of  changed 
limestone.  The  illustration  shows  the  small  area  of  this  zone  where  portions  of 
the  limestone  have  been  changed  to  a  dark  mass  of  minerals.  (Photo  by 
Paige,  U.  S.  Geological  Survey.) 

Fig.  18,  where  a  dike  in  Pennsylvania  has  penetrated  shales  and  shaly  sandstones. 
On  each  side  of  the  old  dike  the  reddish  shales  have  been  baked  to  a  bluish  color 
and  have  been  made  somewhat  hard  and  slaty.  The  soils  from  this  metamorphosed 
zone  are  bluish  in  color  and  contain  fragments  of  rock  in  contrast  to  the  reddish  soils 
from  the  unaffected  sandstones. 

Regional  metamorphism  is  associated  with  the  great  pressures 
involved  in  extensive  earth  movements,  such  as  folding  and  warping  of 
the  earth's  crust.  Rocks  are  often  so  completely  changed  that  it  is 
extremely  difficult,  if  not  impossible,  to  ascertain  their  original  con- 
dition. Often  when  a  mass  of  stratified  rock  has  been  subjected  to 
metamorphism,  it  is  found  that  the  rocks  more  susceptible  to  these 
processes  will  become  metamorphosed  while  others  are  apparently  but 
little  affected. 


52  ROCKS 

Some  of  these  features  are  illustrated  in  Fig.  44,  which  shows  a  section  of  closely 
folded  rocks  in  the  Green  Mountains  of  Massachusetts.  It  is  uncertain  as  to  the 

former  condition  of  the  gneiss  (G)  for 
tne    rock    nas    been    so    thoroughly 
changed.    The  Vermont  formation  (F) 
shows  interesting  transformations.     In 
FIG.  44.-Section  of  metamorphosed  rocks   placeg  it  ig  ft  metamorphic  conglome- 
in.  the  Green  Mountains,  Mass.     (U.  S.   rate>  the  crughed  and  broken  pebbleg 
Geological  Survey.)  of  which  bear  evidence   to   the  great 

stresses  to  which  the  rock  has  been 

subjected.  The  limestones  (L)  have,  for  the  most  part,  been  changed  to  marble 
which  often  contains  mica.  A  careful  study  of  these  formations  shows  that  they 
have  been  derived  from  sedimentary  rocks.  As  proof  a  few  fossils  have  been 
found  and,  moreover,  it  has  been  possible  in  places  to  trace  a  given  formation  from 
metamorphosed  rock  to  limestones,  sandstones  or  shales. 

Kinds  of  Metamorphic  Rocks 

Metamorphic  rocks  are  commonly  divided  for  description  into  two 
classes,  the  foliated  (banded)  and  the  non-foliated.  The  foliated  rocks 
include  gneiss,  schist  and  perhaps  slate,  although  the  banding  in  slates 
is  obscure.  The  non-foliated  rocks  include  marble  and  quartzite. 

Gneiss  (pronounced  nice).  These  rocks  are  coarsely  banded 
with  the  bands  commonly  of  different  minerals,  Fig.  40.  Very  often 
the  light-colored  bands  contain  quartz  and  feldspar,  and  the  dark- 
colored  bands,  the  ferro-magnesian  minerals.  Such  an  appearance 
has  given  rise  to  the  common  name,  "  banded  granite."  There  is  con- 
siderable variation  in  composition  although  most  of  the  gneisses  corre- 
spond to  the  acid  igneous  rocks  of  which  granite  and  syenite  are  exam- 
ples. They  are  usually  somewhat  hard,  durable  rocks  but  their  easy 
splitting  renders  them  unfit  for  building  purposes  where  they  must 
sustain  considerable  weight.  They  are  almost  invariably  found  in 
regions  of  regional  metamorphism  where  the  rocks  have  been  subjected 
to  great  pressure  and  folding.  Sandstones,  some  shales,  granites 
and  yet  other  rocks  have  been  traced  into  regions  where  they  have  been 
changed  to  gneisses.  According  to  the  predominating  minerals  there 
are  biotite  gneiss,  hornblende  gneiss,  etc.  The  structure  of  gneisses 
causes  them  to  break  down  into  soil  rather  readily  and  they  are  impor- 
tant soil  formers  in  the  Piedmont  Plateau  of  the  eastern  United  States. 

Schists  are  finely  foliated  rocks,  usually  dark  colored  and  in  general 
they  contain  less  quartz  and  feldspar  fend  are,  therefore,  more  basic 
than  gneisses.  Shales,  diorites  and  schists  commonly  have  much  the 
same  composition,  and  in  general,  schists  correspond  in  composition 


KINDS  OF  METAMORPHIC  ROCKS 


53 


to  the  basic  igneous  rocks.  Both  local  and  regional  metamorphism 
produce  schists.  They,  accordingly,  have  a  wide  distribution  and  vary 
greatly  in  composition.  Mica  is  a  very  common  mineral  in  schists,  but 
a  great  variety  of  minerals,  including  garnet,  quartz,  feldspars,  horn- 
blende, talc  and  many  other  minerals  are  found  in  these  rocks.  As  in 
gneisses  the  predominating  mineral  gives  its  name  to  the  schist,  and  as 
the  feldspar  and  quartz  increase  and  the  banding  becomes  coarser 
the  schists  grade  into  gneisses. 

Schists  are  seldom  hard  or  durable.  Their  easy  cleavage,  softness 
and  weakness  prevent  their  use  for  road  metals,  building  and  for  other 
structural  purposes.  Locally  they  yield  garnets,  corundum  and  other 
minerals.  Owing  to  their  weakness,  schists  readily  break  up  into  smaller 
pieces  but  do  not  so  readily  decompose  into  soils  and  the  prevailing  mica 
often  causes  the  soils  to  be  highly  micaceous.  The  general  lack  of 
feldspars  causes  soils  derived  from  schists  to  be  somewhat  poor  in  lime 
and  potash.  Since  they  are  relatively  weak  rocks,  schists  usually  wear 
down  to  lowlands  while  adjacent  gneisses  commonly  make  highlands. 

Slates  have  the  well-known  quality  of  easy  splitting.     This,  as  has 
been  noted,  is  due  to  the  growth  of  very  small  minerals,  mostly  mica, 
which   lie   parallel  to  each  other 
and  thus   facilitate  splitting  and 
give  the  dull  silvery  sheen  to  be 
seen  on    smooth    slate    surfaces. 

In  composition  and  often  in  ap-  |yii  \  v 

pearance,    slates    often    resemble 
shales  and,  in  fact,  easy  splitting 
shales  are  often  incorrectly  termed 
slates;  the  geological  usage  is  to 
restrict  the  term  to  metamorphosed 
rocks  only.    They  are  in  most  cases 
the  product  of  regional  metamor- 
phism so  that  the  productive  slate 
belts   of   Vermont,    Pennsylvania 
and  Virginia  lie  in  the  folded  rocks  FlG-  45.-Slate  developed  from  shale  by 
of  the  Appalachians  and  the  slate  metarnorphism.    (Dale,  U.  S.  Geological 
fields  in  Wales  lie  in  regions  of      Survey). 
intensely  folded  rocks. 

Slates  are  typically  changed  from  shales  and  the  lines  of  cleavage 
developed  in  slate  may  or  may  not  coincide  with  the  bedding  planes  of 
the  shale.  It  will  be  clear  from  Fig.  41  that  these  planes  will  not  coin- 


54  ROCKS 

tide  unless  the  pressures  are  about  at  right  angles  to  the  bedding  planes. 
Fig.  45  shows  old  bedding  planes  folded  downward  while  the  main  slaty 
cleavage  planes  are  straight  and  do  not  at  all  correspond  to  the  bedding 
planes.  In  some  slates  the  old  bedding  planes  are  now  represented  by 
dark  bands  or  "  ribbons." 

The  colors  range  from  blues,  reds,  brown,  purple,  black  to  gray. 
The  hardness  and  easy  cleavage  make  them  well  adapted  for  roofing. 
As  a  rule  slates  break  easily  into  thin  pieces  under  weathering  but  their 
further  change  is  somewhat  slow,  so  that  soils  derived  from  slate  usually 
contain  many  fragments,  and  slates,  therefore,  commonly  yield  slate 
loams.  Such  soils  often  extend  in  long  narrow  belts  which  follow  the 
slate  outcrops. 

Quartzite  is  a  metamorphosed  sandstone  or  less  frequently  a  meta- 
morphosed conglomerate.  When  the  grains  of  a  sandstone  are  so 

thoroughly  cemented    by    silica 
that  the  whole  rock    is  a  solid 
mass    the  rock  is   a   quartzite. 
This  cementation  may  and  often 
does  occur  as  a  result  of  local 
and  regional  metamorphism,  but 
it  also  occurs   in    recent   undis- 
turbed sand  and  gravel.     There- 
fore     quartzite,     unlike     other 
metamorphic     rocks,     does    not 
require  earth  movements  for  its 
formation,  but  is  to  be  explained 
by  water  deposition  of  silica  in 
FIG.  46.-Quartzite.      The  grains  (darker  the  interstices  of  sandstones  and 
masses)  have  been   cemented    by  silica,    conglomerates.     The  sand  grains 
Microphotograph,  much  enlarged.  (Mary-   are    not    ordinarily    distinct    to 
land  Geological  Survey.)  the  unaided  eye,  but  the  micro- 

scope shows   the    original    sand 

grains  surrounded  by  deposits  of  silica  which  binds  them  into  an 
exceedingly  hard  rock,  as  is  shown  in  Fig.  46.  Quartzite  ordinarily 
breaks  with  a  conchoidal  fracture  and  is  usually  so  firmly  cemented  that 
the  rock  will  break  across  the.  original  sand  grains  rather  than  around 
them.  There  is  naturally  no  sharp  distinction  between  sandstone  and 
quartzite  and  intermediate  rocks  are  often  termed  "  quart zitic  sand- 
stones "  or  "  quartzitic  conglomerates."  On  the  other  hand,  quartzites 
grade  into  gneisses  or  schists  when  they  have  been  subjected  to  pressure 


KINDS  OF  METAMORPHIC  ROCKS 


55 


sufficient  to  produce  schistosity  and  also  when  there  are  sufficient  mate- 
rials for  the  development  of  feldspars,  micas  and  other  metamorphic 
minerals. 

Quart zites  have  a  peculiar  glazed  or  glassy  luster  and  are  commonly 
gray  or  white  in  color  although  small  amounts  of  iron  oxides  will  impart 
reddish  or  brownish  tints.  Their  extreme  hardness  prevents  much  use 
of  the  rock  in  structural  work  because  of  the  difficulty  in  working  it. 
Naturally  quartzite  breaks  up  very  slowly  under  the  attack  of  the 
weather.  Typically  it  makes  high,  rugged  country  with  but  few  areas 
of  arable  soils. 

Marble. — This  well-known  rock  results  from  the  metamorphism  of 
limestone  or  dolomitic  limestone.  It  may  result  from  either  local  or 


FIG.  47. — The  limestone  on  the  left  has  been  changed  to  the  marble  on  the  right. 
Microphotograph.     (U.  S.  Geological  Survey.) 

regional  metamorphism  and  in  the  formation  of  marble  water  is  an 
important  factor,  for  it  dissolves  and  recrystallizes  the  rock.  For 
example,  Fig.  47  shows  a  somewhat  impure  limestone  with  an  irregular 
structure  and  the  same  limestone  is  seen  changed  to  marble.  The  lime- 
stone has  been  dissolved  and  recrystallized  into  the  large  grains  of  the 
marble.  The  great  pressure  to  which  many  marbles  have  been  sub- 
jected has  seldom  produced  the  schistosity  in  them  that  is  so  well  seen 
in  gneisses  and  schists  and  this  absence  is  probably  to  be  explained  by 
the  fact  that  calcite  and  dolomite,  the  essential  minerals,  easily  dissolve 
and  recrystallize  under  pressure. 

Pure  limestone  will  change  into  a  pure  marble,  but  impure  limestones 
afford  materials  for  many  metamorphic  minerals  and  change  into  com- 


56 


ROCKS 


plex  metamorphic  rocks.  The  hardness  of  pure  marble  is  that  of  cal- 
cite.  Marble  is  used  for  building  and  decorative  purposes  because  of 
its  easy  working,  strength  and  pleasing  appearance.  Inferior  marbles 
are  sometimes  burned  for  lime.  As  a  trade  name  marble  includes  all 
calcareous  rocks  which  will  take  a  pleasing  polish  and  are  of  commercial 
value  and,  indeed,  there  is  no  sharp  division  between  marble  and  lime- 
stone. As  a  soil  builder,  marble  behaves  essentially  as  limestone  but 
the  areas  are  small  and  unimportant. 

REFERENCES 

J.  F.  KEMP,  Handbook  of  Rocks,  Van  Nostrand,  1911,  Chapters  9-12.  The  Meta- 
morphic Rocks. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  pages 
135-149.  (Metamorphic  Rocks.) 

STRUCTURES  COMMON  TO  ALL  ROCKS 

Nearly  all  consolidated  rocks  and  even  many  clays  are  intersected 
by  crevices  or  joints  which  are  usually  more  or  less  perpendicular  to  the 
surface,  Fig.  48;  to  a  less  extent,  joints  are  found  extending  parallel  to 


FIG.  48. — Vertical  and  horizontal  joints  in  granite,  Conn.    (U.  S.  Geological  Survey.) 

the  surface.     Joints  are  often  roughly  parallel  to  each  other  and  so 
form  joint  systems.    Moreover  two  or  more  joint  systems  often  inter- 


STRUCTURES  DUE  TO  FOLDING  57 

sect  at  various  angles,  thus  dividing  the  rocks  into  rough  blocks  and 
often  presenting  a  curious  effect  of  masonry.  Bituminous  coal  is 
frequently  intricately  jointed  and  so  breaks  readily  into  small  blocks, 
and  quarrying  of  dimension  stone  is  greatly  facilitated  by  joints  because 
blocks  can  thereby  be  easily  taken  out.  From  another  point  of  view, 
jointing  is  very  important  in  allowing  the  entrance  of  weathering 
agents  whereby  the  rocks  are  broken  up  and  ultimately  changed  to  soils. 

Structures  Due  to  Folding 

While  many  rocks  appear  to  be  horizontal  in  ordinary  sections,  yet 
observation  shows  that  many  rocks  are  arranged  in  folds  or  wrinkles, 
either  large  or  small.  These  folds  in  part  explain  why  it  is  a  common 
experience  in  nearly  all  mining  and  well  drilling  that  certain  strata  lie  at 
different  depths — nearer  the  surface  in  the  upfolds  and  deeper  in  the 
downfolds. 


FIG.  49. — Generalized  diagram  showing  structure,  topography  and  soils  of  folded 
rocks  in  northern  Georgia. 

Rocks:  (SS),  sandstone;  (LS),  limestone;  (CH),  cherty  limestone;  (S),  shale;  (!TD),Knox 
dolomite,  a  notable  soil  making  formation  here.  Soils:  (L),  loam;  (STL),  stony  loam;  (GL), 
gravelly  loam;  (SHL),  shale  loam;  (C),  clay;  (SL),  silt  loam;  (CL),  clay  loam;  (FSL),  fine 
sandy  loam;  (R),  stony  soils.  (U.  S.  Geological  Survey;  U.  S.  Bureau  of  Soils.) 

It  will  be  seen  from  Fig.  49  that  folding  has  two  important  effects;  it 
brings  different  rocks  to  the  surface  and  also  repeatedly  brings  up  the 
same  formation.  It  will  also  be  noted  that  folded  rocks  outcrop  in 
more  or  less  parallel  belts.  These  belts  in  weathering  to  soils  give 
more  or  less  parallel  soil  belts  which  are  shown  in  the  same  diagram.  A 
study  of  these  soils  shows  that  the  soil  belts  do  not  always  correspond 
to  the  different  geological  formations  exposed,  since  some  adjoining 


58 


ROCKS 


FIG.  50. — To  illustrate  dip,  strike  and 
outcrop. 

in  composition.  Thus  it  is  not  safe 
to  assume  that  soils  in  various 
localities  are  the  same  because  they 
are  derived  from  the  same  forma- 
tion. However,  the  soil  belts  from 
a  single  formation  are  usually 
somewhat  similar. 

Dip  and  Strike. — At  this  point 
it  will  be  useful  to  introduce  terms 
used  in  describing  rocks  that  are 
not  horizontal.  The  angle  that  a 
stratum  or  formation  makes  with 
the  horizon  is  termed  the  dip.  If 
a  stratum  is  vertical  the  dip,  of 
course,  is  90°  and  from  this  the 
dip  or  a  stratum  may  vary  to  a 
horizontal  position  when  the  dip  is 
zero.  The  direction  of  the  dip  is 
also  given;  for  example,  a  stratum 
may  dip  15°  northwest.  Strike  is 
the  imaginary  line  where  a  dipping 
stratum  intersects  the  plane  of  the 
horizon,  Fig.  50;  it  is  perpendicular 
to  the  dip.  It  will  further  be 
evident  that  dip  is  expressed  in 
degrees  and  direction  while  strike 
is  expressed  in  direction.  For 
example,  a  stratum  may  dip  20° 
west  and  strike  north-south.  It 
is  important  to  note  the  relation 


formations  do  not  differ  ma- 
terially except  that  they  con- 
tain different  fossils  and  so  are 
classed  as  different  formations. 
On  the  other  hand,  a  single 
formation  may  yield  different 
soil  types  at  different  places 
because  of  surface  differences 
of  drainage  or  erosion  or 
because  of  some  differences 


FIG.  51. — Diagram  illustrating  the 
changing  width  of  outcrop  due  to 
variations  in  dip. 


STRUCTURES  DUE  TO  FOLDING 


59 


FIG.  52. — Anticline  of  sandstone,  Md.  (U.  S. 
Geological  Survey.) 


between  dip  and  width  of  outcrop.  For  example,  in  Fig.  51  consider  a 
stratum  with  uniform  thickness  to  be  dipping  at  various  angles.  When 
the  stratum  is  vertical  (A)  the  outcrop  is  as  wide  as  the  stratum  is  thick 
and  in  this  position  the  outcrop  is  narrowest.  As  the  dip  decreases 
(B  and  C)  the  width  of  the 
outcrop  increases  until  the 
maximum  width  of  outcrop 
(D)  occurs  when  the  stratum 
is  horizontal.  With  this  in 

mind  it  is  clear  how  a  given         BS^S  HI 

dipping  stratum,  say  of  lime- 
stone, may  in  one  place  yield 
a  soil  belt  three  miles  wide 
while  in  another  place  the 
width  may  be  more  or  less 
according  to  the  dip. 

Anticline  and  Syncline. — An  anticline  is  an  arch-like  upward  fold, 
Fig.  52,  with  the  rocks  dipping  down  on  either  side  from  the  center.  A 
syndine  is  the  reverse  of  the  anticline,  being  a  trough-like  fold  with  the 

rocks  dipping  inward  towards 
the  center,  Fig.  53.  Anti- 
clines and  synclines  usually 
occur  together.  Sometimes 
the  folding  is  open  and  again 
the  folds  are  closely  folded 
and  very  complex. 

An    important    belt   of   folded 
rocks  is  found  in  the  Eastern  United 
States,  Fig.  54,  which    practically 
extends    from    Nova    Scotia    into 
FIG.  53.— Syncline  of  shale,  Pa.  Alabama.      Especially    notable    in 

(U.  S.  Geological  Survey.)  this  folded  belt  is  a  wide>  rather 

level  floored  valley  which  extends 

from    New   Jersey   into    Alabama 

under  various  local  names  such  as  the  Kittatinny  Valley  in  New  Jersey,  the 
Lebanon  and  Cumberland  Valleys  in  Pennsylvania,  the  Shenandoah  Valley  in 
Virginia  and  the  Tennessee  Valley  farther  south.  Folding  has  upraised  a  belt  of 
limestone  and  shales  which  underlie  the  valley  and  yield  the  well-known  fertile  soils. 
A  section  across  this  valley  in  Pennsylvania  appears  in  Fig.  55.  The  limestones 
and  shales  yield  respectively  the  Hagerstown  and  Berks  soil  series.  On  the  east  is 
South  Mountain,  a  group  of  hills  carved  from  sandstone  and  quartzite,  and  on  the 
west  are  the  ridges  of  the  Ridge  Belt.  Folding  of  all  degrees  is  found  in  the  Cor- 
dilleran  Mountains  of  the  West.  The  Alps  are  also  notable  examples  of  folding. 


60 


ROCKS 


lAi.TfMflRVr-W;-     ^         ./..•         T 

^\-M^v./  y 

^^in^Td/)  i^H  ^ •  V         ° 


FIG.  54. — A  part  of  the  Appalachian  Ridge  Belt.      (U.  S.  Geological  Survey.) 


APPALACHIAN  ~- 

^IDGES_  CUMBERLAND  VALLEY    HAGERSTOWN 

BERKS  SOILS 


FIG.  55. — Diagram  to  show  rock  structure  and  topography  of  the  Cumberland 
Valley  and  South  Mt.  in  Pennsylvania.  (Data  after  U.  S.  Geological  Survey 
and  U.  S.  Bureau  of  Soils.) 


TOPOGRAPHY  PRODUCED  BY  FOLDING  61 

Topography  Produced  by  Folding 

Not  only  must  the  residual  soil  belts  produced  by  folding  be  con- 
sidered, but  also  the  topography,  for  topography  is  perhaps  as  important 
a  soil  factor  as  the  parent  rocks.  When  strong  and  weak  rocks  are 
exposed  to  erosion  under  the  same  conditions,  the  weak  rocks  will  wear 
faster  and  produce  a  lower  area  than  the  area  underlain  by  strong  rocks. 
In  other  words,  the  strong  rocks  make  the  higher  and  the  weak  rocks 
the  lower,  country. 

It  is  somewhat  natural  to  think  of  anticlines  as  ridges  and  synclines 
as  valleys  and  such  is  often  the  case,  especially  with  rocks  that  were 
recently  and  rapidly  folded.  But,  on  the  other  hand,  where  the  folding 
was  ancient  and  the  rocks  have  been  long  exposed  to  erosion  as  in  the 
Appalachian  Mountains,  the  reverse  is  often  the  case.  A  somewhat 
ideal  case  is  illustrated  in  Fig.  56,  where  a  homogeneous  rock  such  as, 
for  example,  a  granite,  has 
been  folded.  It  will  readily 
be  seen  that  the  rocks  in 
the  upper  parts  of  anti- 
clines are  stretched  and 
tend  to  be  shattered  and 
weakened.  On  the  other  FIG.  56. — Diagram  to  illustrate  the  evolution  of 
hand,  the  rocks  in  the  valleys  on  anticlines, 

synclines,  especially  in  the 

lower  parts,  are  somewhat  compressed  and  less  weakened.  As  a 
result  the  anticlines  wear  away  faster  and  become  valleys  and 
the  synclines,  being  relatively  less  eroded,  are  left  as  ridges.  The 
development  of  anticlines  into  valleys  and  synclines  into  mountains 
finds  an  interesting  illustration  in  the  anthracite  coal  field  of  Penn- 
sylvania. The  coal  overlying  the  anticlines  has  often  been  removed  by 
erosion  while  the  coal  in  the  synclines  has  often  been  preserved. 

As  a  matter  of  fact,  such  simple  conditions  are  rarely  found.  The 
most  characteristic  topography  resulting  from  folding  is  found  in  regions 
of  folded  sedimentary  rocks.  Here  the  rocks  of  various  strengths  are 
exposed  with  the  result  that  the  weaker  rocks  are  worn  to  valleys,  while 
the  relatively  strong  rocks  form  ridges  as  shown  in  Fig.  57.  The 
upturned  strata  form  rather  straight,  even  crested  ridges  which  when 
followed  for  some  distance,  usually  curve  and  finally  disappear  as  the 
rock  may  become  relatively  weak.  Between  the  ridges  are  rather  narrow 
valleys  in  which  most  of  the  arable  lands  and  population  are  located. 


62 


ROCKS 


Blue  Mountain  in  Pennsylvania  is  one  of  these  ridges,  Fig.  58;  it  is  due 
to  resistant  sandstone  and  extends  with  fairly  even  crest  for  scores  of 
miles.  Thus  the  streams  and  their  valleys  usually  map  the  weak  rocks 


FIG.  57. — Diagram  to  illustrate  the  development  of  ridges  and  valleys  on  folded 
rocks.  The  strong  rocks  (S)  make  the  ridges  and  the  weak  rocks  (W),  the 
valleys.  The  dotted  lines  indicate  the  eroded  strata. 


FIG.  58. — Ridges  caused  by  folding. 
Distant  view  above;  the  first  ridge 
is  Blue  Mountain.  The  Diagram 
below  shows  four  ridges  made  by 
sandstones  (S)  which  have  been 
upturned  by  folding.  The  arrow 
shows  the  position  of  the  camera. 
The  two  ridges  in  the  foreground  of 
the  diagram  are  those  shown  in  the 
photograph.  Penna. 

and  the  ridges  map  the  strong  rocks,  both  valleys  and  ridges  being  more 
or  less  parallel.  So  characteristic  is  this  parallel  arrangement  of  streams 
in  folded  rocks  that  such  structure  can  sometimes  be  detected  from 
ordinary  maps. 


FAULTS 


63 


The  "Blue  Grass0  Basins  of  Kentucky  and  Tennessee  are  examples  on  a  large 
scale  of  eroded  anticlines,  Fig.  59.  An  extensive  but  low  anticline  was  uplifted 
and  its  upper  portion  has  been 
eroded.  Hundreds  of  feet  of  over- 
lying rock  have  been  removed  so 
that  the  underlying  limestone  in 
the  anticline  has  been  exposed 
and  the  limestone  now  affords  the 
famous  productive  soil  of  these 
regions.  The  "Highland  Rim" 
which  surrounds  the  basin  has  not 
been  eroded  so  fast.  Its  sandstones 
and  shales  afford  less  productive 


soils  and,  moreover,  the  surface  has 
been  eroded  to  a  hilly  topography. 


FIG.  59. — Diagram  of  the  Blue  Grass  and 
Highland  Rim  regions  of  Tennessee. 

Faults 


A  close  examination  in  almost  any  quarry  will  usually  show  small 
cracks  along  which  the  rocks  have  slipped,  Fig.  60.     Such  breaks  where 

rocks  have  slipped  so  that 
formerly  continuous  strata  no 
longer  match  are  termed 
faults.  It  should  be  noted 
that  not  all  cracks  are  faults; 
only  when  movement  has 
^occurred  along  the  breaks  is 
the  term  applied.  Faults  of 
various  magnitudes  are  much 
more  common  than  might  be 
supposed  since  many  faults 
are  concealed  and  many  others 
are  hard  to  detect.  Some 
terms  are  convenient  in  the 
description  of  faults.  Fig.  61 
shows  a  fault  in  which  the 

ilSifiSl^P    strata  ^ on  the  right  have 

been  elevated  relative  to  those 
••Hw  on  the  left  and   consequently 


FIG.  60.— Small  faults,  Texas;  the  strata  do 
not  match.     (U.  S,  Geological  Survey.) 


the  right  side  is  termed  the 
upthrow  side  and  the  left,  the 
downthrow  side.     These  terms 
are  merely  relative  and  do  not  imply  that  the  rocks  on  the   right 


64 


ROCKS 


were  actually  pushed  up   or  the  rocks 


FIG.  61. — Diagram  to  illustrate  a  fault. 


on  the  left  depressed;  the 
arrows  show  the  relative 
movements. 

Angle  B AC  is  termed 
the  hade  of  the  fault.  The 
vertical  displacement  A  B 
is  termed  the  throw,  the 
horizontal  displacement  BC, 
the  heave,  while  the  move- 
ment along  the  fault  AC  is 
the  displacement. 


Effects  of  Faulting 

Faults  of  much  magnitude  rarely  occur  singly,  but  usually  in  a  group 
of  more  or  less  parallel  faults,  the  fault  zone.  Faults  are  naturally  more 
numerous  in  folded  rocks  where  the  rocks  have  been  stressed  and  broken 
by  the  enormous  strains  to  which  they  were  subjected.  From  an  agri- 
cultural point  of  view,  faults  are  interesting  both  from  their  direct  and 
their  indirect  effects  on  soils.  The  direct  effect  is  seen  when  faulting 
brings  up  on  the  upthrow  side  a  rock  which  yields  soils  different  from 
what  they  would  otherwise  have  been.  The  indirect  effects  are  due  to 
the  influence  of  faulting  on  topography.  Soils  on  the  upthrow  side  are 
often  higher,  and  t  herefore 
exposed  to  greater  erosion; 
they  are  likely  to  be  thinner 
and  often  more  stony  as  a 
result  of  the  upthrow.  Soils 
on  the  downthrow  side  are 
likely  to  be  lower,  less  actively 
eroded  and  to  have  less  active 
drainage  as  results  of  the  FIG.  62. — Diagram  showing  an  effect  of  faulting 
downthrow.  on  so^s-  The  limestone  on  the  left  has  been 

brought    up    by  faulting   and   yields    stony 

Both  of   these  effects   are   seen       soils,  Texas.     (Data  after  Hill,  U.  S.  Geo- 
in   central   Texas,    Fig.  62.     West       logical  Survey.) 
of  Austin   is   a   series  of  faults,,  a 

fault  zone,  running  in  a  north-south  direction  with  the  upthrow  side  on  the  west. 
Between  the  upthrow  and  downthrow  sides  is  a  steep,  eroded  slope  (escarpment) 
known  locally  as  "The  Balcones."  The  lower,  rolling  country  is  a  part  of  the 
Coastal  Plain;  the  high  country  is  known  locally  as  the  "mountains"  or  the 
Edwards  Plateau.  This  plateau  because  of  its  height  has  been  eroded  so  that 


EFFECTS  OF  FAULTING  65 

the  topography  is  for  the  most  part  very  hilly  and,  as  a  result,  much  of  the 
soil  has  been  washed  away,  leaving  the  remaining  soils  thin.  The  underlying  rock 
is  hard  flinty  limestone.  As  a  whole  the  soils  are  stony  loams.  The  lower,  level 
Coastal  Plain  is  much  less  eroded.  The  black  clay  soils  are  derived  largely  from  a 
marly  clay.  The  clay  soils  are  largely  derived  from  a  chalky  limestone. 

It  is  natural  to  think  of  the  upthrow  side  as  forming  a  steep  slope  (escarpment) 
near  the  fault  and  when  this  oc- 
curs, the  slope  is  termed  a  fault 
scarp.  Such  a  fault  scarp  extend- 
ing for  miles  is  found  in  central 
Texas,  Fig.  62.  A  recent  fault 
scarp  is  shown  in  Fig.  63.  A  fault 

scarp  may  be  made  by  an  upthrow  FlG'  <B.-Faiilt  scarp  in  Arizona.  Land 
which  elevates  one  side  but  such  waste  has  accumulated  at  the  base  of  the 
a  scarp  is  likely  to  be  worn  down  *carP  an(J  conceals  the  lower  ^  The 
and  obliterated  by  erosion  in  a  com-  diaSram  shows  the  stru<;ture-  (Ransome, 
paratively  short  time  as  time  is  IL  S'  Geol°Slcal  Survey-) 
reckoned  in  geology.  On  the  other 

hand,  if  the  upthrow  side  is  composed  of  relatively  resistant  rock  the  fault  scarp 
may  persist  for  a  long  time. 

The  eastern  slopes  of  the  Sierras  consist  of  a  series  of  great  fault  scarps  with 
throws  of  hundreds  of  feet.  Fresh  fault  scarps  are  found  in  New  Mexico  and  Ari- 
zona which  extend  practically  unbroken  for  long  distances.  Block  Mountains  are 
common  in  the  great  Basin.  They  are,  as  the  term  implies,  blocks  w^hich  have  been 
elevated  and  tilted  so  as  to  constitute  elevations  above  the  general  surface.  It 
should  be  noted  that  all  great  earthquakes  are  thought  to  be  connected  with  faulting. 
The  California  earthquake  in  1906  is  a  case  in  point.  Movements  along  an  old  fault 
sent  vibrations  through  the  rocks  which  produced  the  earthquakes. 

GENERAL  REFERENCES  ON  ROCKS 

CHAMBERLIN  AND  SALISBURY,  Geology,  Vol.  1,  Chapter  7. 

J.  F.  KEMP,  Handbook  of  Rocks,  Van  Nostrand,  1911,  Chapter  1,  Introduction  to 

the  Study  of  Rocks. 
L.  V.  PIRSSON,  Rocks  and  Rock  Minerals,  Wiley,  1908. 

FOLDED  ROCKS 

C.  W.  HAYES,  The  Southern  Appalachians  in  Physiography  of  the  United  States, 

American  Book  Co.,  1895,  pages  305-336! 
BAILEY  WILLIS,  Mechanics  of  Appalachian  Structure,  13th  Ann.  Rept.,  Part  2,  U.  S. 

Geological  Survey,  1892. 
The  Northern  Appalachians  in  Physiography  of  the  United  States,  American  Book 

Co.,  1895,  pages  69-202. 
C.  F.  MARBUT,  Reconnaissance  Soil  Survey  of  the  Ozark  Region  of  Arkansas  and 

Missouri,  U.  S.  Bureau  of  Soils,  1911.     (Anticlinal  structure.) 
U.  S.  Bureau  of  Soils,  Reconnaissance  Soil  Survey  of  South-Central  Pennsylvania, 

1910;  Southwestern  Pennsylvania,  1909. 


66  ROCKS 


FAULTS 

Muscogee  Folio,  Okla.,  U.  S.  Geological  Survey,  1906.     Compare  with  Soil  Survey  of 

Muscogee  Co.,  Okla.,  U.  S.  Bureau  of  Soils,  1913. 
Austin  Folio,  Tex.,  U.  S.  Geological  Survey.     Compare  with  Soil  Surrey  of  Austin 

Area,  Tex.,  U.  S.  Bureau  of  Soils,  1904. 
Uvalde  Folio,  Tex.,  U.  S.  Geological  Survey,  1900. 
Tishomingo  Folio,  U.  S.  Geological  Survey.    Compare  with  Soil  Survey  of  Tishomingo 

Co.,  Okla    U.  S,  Bureau  of  Soils,  1906. 


CHAPTER   III 
WEATHERING 

Erosion  is  the  general  process  by  which  all  land  surfaces  are  being 
broken  down,  carried  away  and  deposited  in  the  ocean.  This  process 
is  so  slow  that  it  can  be  observed  in  notable  action  only  in  exceptional 
instances,  yet,  during  geological  time,  the  lands  have  been  repeatedly 
elevated  and  worn  down.  It  is  a  world-wide  process,  ever  in  operation 
and  shaping  the  earth's  surface. 

Erosion  may  be  considered  under  four  heads :  weathering,  the  process 
by  which  the  earth's  crust  is  broken  up  so  that  it  can  be  carried  ocean- 
ward;  transportation,  by  which  the  comminuted  rock  is  carried;  corro- 
sion refers  to  the  wear  and  tear  of  rock  materials  carried  by  streams ; 
deposition  applies  to  the  final  laying  down  of  the  stream  loads. 

PROCESSES  OF  WEATHERING 

Weathering. — It  is  a  matter  of  common  observation  that  exposed 
rocks  in  time  become  less  firm,  sometimes  change  in  color  and  finally 
are  broken  down.  Smoothed  surfaces  of  old  monuments  lose  their 
polish  and  old  and  fresh  rock  surfaces  in  quarries  are  different.  Weather- 
ing is  the  general  term  applied  to  the  combination  of  processes  by  which 
rocks  are  broken  down  so  that  they  can  be  carried  seaward.  The  term 
was  first  applied  to  the  action  of  frost  and  other  atmospheric  agents 
but  is  now  used  to  include  all  processes  by  which  rock  is  broken  down. 
A  rock  is  sometimes  broken  down  with  little  change  in  its  composition, 
a  process  termed  disintegration;  for  example,  when  water  freezes  and 
pries  open  a  rock,  there  is  little  or  no  chemical  effect  of  the  water.  On 
the  other  hand,  when  a  rock  like  granite,  for  instance,  is  not  only  broken 
into  fragments  by  freezing  and  other  processes  of  disintegration,  but  the 
composition  of  the  rock  is  changed,  the  process  is  termed  decomposi- 
tion. Disintegration  and  decomposition  practically  always  work 
together  although  one  process  is  usually  dominant  at  a  given  time  and 
place;  the  one  process  is  physical,  the  other  chemical. 

67 


68 


WEATHERING 


FIG.  64. — Gradations  from  limestones  below 
to  soils  above,  Kansas. 


Residual  Soils. — The  soils  due  to  weathering  are  termed  residual 
soils  because  they  are  residues  from  which  the  soils  are  derived.  A 

residual  soil  is  a  changing  zone. 
The  upper  portions  are  being 
washed  away  while  fresh  soil 
is  being  made  from  the  rock 
below,  as  shown  in  Fig.  64, 
where  the  underlying  rock  can 
be  seen  grading  through  differ- 
ent stages  to  the  fine  soil 
above. 

Processes  of  Decomposition 

Decomposition  is  accom- 
plished by  four  principal  pro- 
cesses, carbonation,  oxidation, 
hydration  and  solution. 

Carbonation  is  a  union  of  carbon  dioxide  (CCfe)  with  a  base,  a  familiar 
example  of  which  is  the  hardening  of  plaster  by  which  the  lime  of 
plaster  combines  with  the  carbon  dioxide  of  the  air  according  to  the 
following  equation: 

Lime        and     Carbon  dioxide     yield     Carbonate  of  lime     and    Water 
Ca(OH)a      +  CO2  CaCO3  +       H2O 

Carbon  dioxide  is  a  gas  which  exists  in  the  atmosphere  in  the  ratio  of 
about  3  parts  of  carbon  dioxide  to  10,000  parts  of  air.  It  is  readily 
soluble  in  water,  forming  carbonic  acid  (H^COs),  so  that  most  rain  water 
and  soil  water  contain  varying  amounts  of  this  acid.  In  rain  water  it 
has  been  found  in  quantities  15  to  40  times  greater  than  in  air.  This 
acid  in  rain  waters  explains  the  active  etching  of  limestones  and  marbles 
by  which  the  carvings  and  lettering  are  slowly  obliterated.  The  car- 
bonic acid  attacks  the  calcite  of  limestones  forming  the  soluble  bicar- 
bonate as  follows: 

Carbonic  acid      acting  on    Calcite     yields     Calcium  bicarbonate 
H2CO3  +          CaCO3        =  H2Ca(CO3)2 

In  the  soil  atmosphere  the  amount  of  carbon  dioxide  may  be  rela- 
tively very  great  especially  if  the  soil  contains  much  organic  matter. 
The  soil  air  in  some  observed  instances  contains  almost  two  hundred 
times  as  much  carbon  dioxide  as  the  atmosphere.  This  is  important 


PROCESSES  OF  DECOMPOSITION  69 

from  an  agricultural  point  of  view,  for  active  carbonation  in  the  soil 
changes  the  lime  and  potash  into  soluble  carbonates. 

Carbon  dioxide  is  emitted  in  enormous  quantities  from  active 
volcanoes ;  it  is  generated  by  fires  and  exhaled  in  the  breath  of  animals. 
Carbonation  is  especially  active  in  warm  moist  regions  because  the 
decay  of  the  rank  vegetation  produces  large  quantities  of  this  gas. 
When  carbonation  occurs  the  resulting  minerals  increase  in  bulk,  thereby 
rupturing  rocks  and  minerals,  making  them  weaker  and  more  porous 
and  aiding  the  different  processes  of  weathering.  Much  of  the  work  of 
carbonation  is  done  by  the  underground  waters,  and  the  process  will  be 
further  considered  under  that  topic. 

REFERENCE 

A  Treatise  on  Metamorphism,  Charles  R.  Van  Hise,  U.  S.  Geological  Survey,  Mono- 
graph 48,  1904,  Carbonation,  pages  473-480. 

Oxidation  is  the  combination  of  elements  and  compounds  with 
oxygen.  Oxygen  is  a  gas  that  exists  in  the  atmosphere  in  the  proportion 
of  about  21  per  cent  of  oxygen  and  78  per  cent  of  nitrogen.  Like 
carbon  dioxide,  but  to  a  much  smaller  extent,  oxygen  is  soluble  in  water 
so  that  rain  and  soil  water  usually  contain  higher  percentages  than  the 
air,  and  as  a  result,  rain  and  ground  water  are  active  agents  of  oxida- 
tion. Oxygen  is  an  active  ele- 
ment and  oxidation  is,  therefore, 
an  active  process.  Active  oxi- 
dation is  conditioned  on  the 
presence  of  moisture,  but  in 
practically  all  parts  of  the  earth 
there  is  enough  moisture  for 
this  process.  The  process  is 
especially  active  in  the  humid 
tropics,  which  are  characterized 
by  much  heat  and  moisture. 

Iron  oxidizes    readily    and, 
since    iron    is     so    widely    dis-   pIG  65.— Weathering  has  etched  out  delicate 
tributed,      the     reddish       iron       structures    in   limestone.     (Phalen,  U.  S. 
oxides   are   much    in   evidence       Geological  Survey.) 
the    world    over.      The     iron 

oxides  often  cover  soil  grains  in  thin  coatings  so  that  the  brilliant  hues 
of  many  rocks  and  soils  are  somewhat  deceptive,  for  when  a  red  rock  is 


70  WEATHERING 

analyzed,  the  proportion  of  iron  is  often  surprisingly  low.  Oxidation, 
like  carbonation,  is  accompanied  by  an  increase  in  bulk,  thus  tending 
to  break  up  the  containing  rock. 

A  common  example  of  oxidation  is  seen  when  pyrite  is  oxidized 
according  to  the  following  reaction : 

Pyrite     and      Oxygen     yield      Iron  sulphate     and    Sulphur 
FeS2        +  40  FeSO4  +          S 

The  iron  sulphate  is  soluble  and  is  often  changed  into  other  com- 
pounds. The  sulphur  may  unite  with  oxygen  and  water  to  form  sul- 
phuric acid.  Some  rocks  contain  considerable  pyrite  and  the  changes 
noted  above  help  to  change  the  rock  into  soil. 

REFERENCE 

A  Treatise  on  Metamorphism,  Van  Hise,  Oxidation,  pages  461-473. 

Hydration  is  the  process  by  which  water  combines  with  various 
compounds  to  make  hydrated  minerals,  a  familiar  example  of  which  is 
the  union  of  lime  with  water  to  form  hydrated  lime  according  to  the 
following  equation : 

Lime       combined  with     Water      yields     Hydrated  lime 
CaO  +  H2O  Ca(OH)2 

Both  in  rocks  and  in  soils  hydration  is  important  and  extensive  espe- 
cially, as  would  be  expected,  in  humid  regions.  The  common  rusting 
of  iron  is  a  familiar  example  of  this  world-wide  process  and  hydrated 
oxide  of  iron  (Fe20s-3H2O)  is  very  common  in  soils,  giving  them  their 
reddish  brown  and  yellowish  colors.  Limonite,  the  hydrated  oxide  of 
iron,  is  an  important  iron  ore.  Hydration,  like  carbonation,  is  accom- 
panied by  an  increase  in  bulk.  Thus,  when  anhydrite  (CaSO/t)  is 
hydrated  and  changed  to  gypsum  (CaSO4-2H20),  the  well-known 
"  land  plaster,"  there  is  so  great  an  increase  in  bulk  that  the  rocks  above 
gypsum  deposits  are  frequently  much  broken  and  folded  because  of  this 
expansion  due  to  hydration.  Merrill  states  that  granites  have  been 
known  to  fall  to  pieces  in  a  few  days  when  removed  from  the  quarry 
because  of  the  hydrated  condition  of  some  of  the  minerals. 

Many  exposed  rocks  have  red  and  yellowish  coatings  of  iron  oxides 
and  hydrates  on  the  outside  while  the  interior  is  unchanged  and  of  the 
original  color.  Thus  a  mass  of  black  rocks  (dike)  in  southeastern  Penn- 
sylvania can  be  traced  for  miles  by  the  reddish  boulders  on  the  surface 


PROCESSES  OF  DECOMPOSITION 


71 


of  the  ground.  The  reddish  and  brownish  stains  of  soils  and  subsoils 
are  important  indications  of  oxidation  and  hydration  by  the  soil  waters 
and  air.  A  poorly  drained  and  poorly  oxidized  subsoil  is  usually  bluish 
or  gray  in  color  and  when  such  soils  have  been  properly  drained  and 
oxidized  the  reddish  and  yellowish  colors  due  to  oxidation  and  hydra- 
tion often  appear. 

REFERENCE 

VAN  HISE,    A  Treatise  on    Metamorphism,    Hydration   and   Dehydration,  pages 
481-483. 

Solution,  as  the  name  indicates,  is  the  dissolving  of  minerals  in  the 
ground  or  soil  water.  Pure  water  is  a  weak  solvent,  even  dissolving 
quartz  and  feldspars  in  very  small  proportions,  but  pure  water  in  rain  or 
ground  water  is  practically  non-existent.  Carbonic  acid,  as  we  have 
seen,  is  practically  always  present  in  rain  and  ground  waters,  and  decay- 
ing vegetation  produces  acids  that  are  effective  solvents.  In  some  por- 
tions of  the  tropics  rain  waters  are  found  to  contain  considerable 
amounts  of  nitric  acid  which  is  believed  to  be  caused  by  lightning. 

A  few  minerals  such  as  common  salt  go  into  solution  unchanged  in 
composition,  but  most  soluble  substances  have  been  chemically  changed 
from  less  soluble  substances;  this  is  especially  true  of  the  carbonates, 
which  are  by  far  the  most  com- 
mon minerals  in  solution  in 
ground  waters.  For  the  most 
part,  solution  is  greatest  in 
regions  of  heavy  rainfall,  which 
both  furnishes  abundant 
ground  and  soil  water  and 
promotes  a  heavy  growth  of 
vegetation  which,  upon  decay- 
ing, yields  acids  to  the  under- 
ground waters.  The  influence 
of  rainfall  upon  solution  is 
especially  well  shown  where 
the  same  soil  types  extend 
from  dry  to  humid  regions. 

For  example,  the  reddish  Orangeburg  soils  extend  through  the  Coastal 
Plain  from  the  Carolinas  into  Texas,  but,  as  they  extend  into  the  dryer 
portions  of  Texas,  their  lime  content  becomes  higher.  Another  inter- 


FIG.  66. — Pitted  limestone  due  to  solution, 
Mo.     (Marbut,  Mo.  Geological  Survey.) 


72 


WEATHERING 


esting  example  is  seen  in  Washington,  where  a  belt  of  fine-grained  soils 
extends  from  a  very  dry  to  a  somewhat  humid  region.  In  the  dry 
region,  the  soils  are  sandy,  but  as  the  rainfall  almost  imperceptibly 
increases  the  soils  change  to  loams  because  the  disintegrated  sand  grains 
decompose  in  part  to  clays. 

The  same  effects  are  illustrated  in  the  table  below.  It  is  usually 
surprising  to  persons  used  to  soils  in  humid  regions  to  observe  the  fer- 
tility of  many  arid  sandy  soils  when  they  are  supplied  with  water. 
However,  it  must  be  remembered  that  this  sand  is  to  a  considerable 
extent  simply  fine  grains  of  rock  and  not  the  residual  sand  of  humid 
regions.  This  is  well  illustrated  by  Hilgard's  comparison  of  soils  sub- 
ject to  different  quantities  of  rainfall,  as  follows:1 


Humid 
regions  average 
of  696  samples, 
per  cent. 

Transition 
region  average 
of  178  samples, 
per  cent. 

Arid 

region  average 
of  573  samples, 
per  cent. 

Insoluble  residue  

88.21 

83.50 

75  87 

Potash 

21 

33 

67 

Soda  

.14 

.32 

.35 

Lime.             *      

.13 

.70 

1.43 

Peroxide  of  iron 

3  88 

2  08 

5  48 

Phosphoric  acid  

.12 

.21 

.16 

Furthermore,  the  rate  of  water  movement  is  an  important  factor 
in  solution.  Evidently  slowly  moving  water  will  dissolve  more  than 
rapidly  moving  water,  other  things  being  equal.  Porosity  of  rock  aids 
soil  solution  to  a  certain  extent  both  by  allowing  a  more  free  movement 
of  water  and  by  exposing  larger  surfaces  to  solution.  The  temperature 
of  water  is  a  somewhat  minor  factor,  but  important  during  long  periods; 
in  general,  the  higher  the  temperature  the  more  active  is  solution,  which 
aids  other  weathering  processes  by  leaving  pores  and  cavities  in  the 
rocks,  Fig.  166. 

REFERENCE 

A  Treatise  on  Metamorphism,  Van  Hise,  Solution,  pages  484-487. 

Association  of  Decomposition  Factors. — It  should  always  be  kept 
in  mind  that  these  processes  of  decomposition  practically  never  act 
except  in  conjunction  with  each  other,  although,  of  course,  one  or  more 
1  E.  W.  Hilgard,  Soils,  page  377,  Edition  of  1911. 


DISINTEGRATION  AND  ITS  PROCESSES  73 

may  be  more  effective  than  others.  Hydration  and  oxidation  almost 
always  act  together  and  solution  and  carbonation  are  closely  con- 
nected. The  cooperation  of  hydration,  carbonation  and  solution  is 
illustrated  in  the  equation  given  below.  The  orthoclase  changes  into 
insoluble  quartz  and  kaolin  and  also  into  the  soluble  carbonate  of 
potash,  which  is  largely  removed  in  solution. 

Orthoclase  and    Water    and  Carbon  yield  Kaolin  and  Quartz  and    Carbonate  of 
feldspar  ^dioxide  potash 

2KalSi3O8    +       2H2O     +      CO2        =  H4Al2Si2O9  +       SiO2     +          K2CO3 

REFERENCES 

E.  W.  HILGARD,  Soils,  Chapter  2,  Decomposition. 

G.  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  1906,  Part  3,  pages  154-158; 
165-174. 

Disintegration  and  Its  Processes 

Disintegration,  as  has  been  noted,  is  the  breaking  down  of  rocks 
by  physical  agents;  it  is  promoted  by  several  factors.  Temperature 
changes  producing  expansion  and  contraction  are  probably  the  most 
widespread  agents  of  disintegration.  The  effectiveness  of  these  agents 
is  well  illustrated  by  the  farmer's  device  of  building  a  fire  on  a  boulder 
and  then  chilling  the  rock  with  water,  when  large  flakes  of  rock  will 
scale  off.  The  same  action  is  seen  in  tropical  regions  when  cool  rain 
falls  on  hot  rock  surfaces.  The  disintegrating  effects  of  temperature 
changes  are  due  to  the  fact  that  all  parts  of  a  rock  do  not  expand  and 
contract  at  equal  rates.  In  other  words  there  is  differential  expansion 
and  contraction  by  which  stresses  are  set  up  which  break  the  rocks.  A 
rock  composed  of  several  minerals  will  be  especially  affected  by  tem- 
perature changes  since  its  minerals  expand  at  different  rates  and,  more- 
over, the  same  mineral  often  expands  at  different  rates  along  certain 
lines  called  axes.  For  example,  quartz  and  hornblende  expand  over  one 
ten-thousandths  of  their  bulk  during  a  temperature  change  of  50°,  a 
small  expansion  to  be  sure,  but  so  effective  on  a  large  surface  that  rock 
has  been  known  to  buckle  and  break.  Thus,  for  example,  when  a  rock 
like  granite  is  heated,  the  closely  packed  minerals  press  against  each 
other  with  great  force  and,  when  the  rock  is  cooled,  the  minerals  shrink 
away  from  each  other;  both  the  contraction  and  expansion  produce 
tiny  fractures  even  in  the  strongest  rocks.  Again,  the  larger  the 
minerals  the  greater  the  mass  expansion;  for  example,  other  things 


74 


WEATHERING 


being  equal,  coarse  granite  would  suffer  more  rupturing  by  temperature 
changes  than  fine-grained  granite,  and  gabbro  than  basalt.  It  must  be 
remembered  that  these  changes,  although  extremely  small,  are  continued 
from  season  to  season  and  often  from  day  to  day.  In  a  long  period  they 
break  the  strongest  rock  into  fragments. 

Rapidity  of  Temperature  Changes. — Disintegration  is  best  pro- 
moted by  rapid  changes  in  temperature  because  the  rocks  have  little 
time  to  become  adjusted  to  the  resulting  strains.  One  can,  for  example, 
bend  steel  or  glass  by  slow  pressure  while  they  would  break  were  the 
pressure  applied  suddenly.  Regions  of  low  humidity  and  high  altitudes 
are  especially  subject  to  sudden  temperature  changes  and  consequently 
disintegration  is  especially  prominent  in  deserts,  in  semi-arid  regions 
and  on  mountain  tops.  In  the  Sahara  the  temperature  differences 
between  night  and  day  are  great  so  that  rocks  are  visibly  ruptured  as 
night  comes  on.  High  mountain  tops  are  nearly  always  covered  with 
coarse,  angular  debris,  for  here  the  processes  of  disintegration  are  dom- 
inant and  decomposition  has  little  opportunity  to  reduce  the  rock 
fragments  to  smaller  and  less  angular  shapes. 

Slower  temperature  changes  are  less  effective  but  are  worldwide. 
The  seasonal  changes  between  summer  and  winter  extending  perhaps 
to  30  or  50  feet  are  effective  in  the  long  run.  Temperature  changes, 
however,  extend  only  to  shallow  depths  as  shown  in  the  following  table, 
which  shows  the  temperature  ranges  in  igneous  rock  (trap)  and  sand- 
stone : 1 


FRAP  ROCK 

SANDSTONE 

Max. 

Min. 

Range. 

Max. 

Min. 

Range. 

3  feet  

52.85° 

38.88° 

13.97° 

53.15° 

38.25° 

14.90° 

6  feet   

51.07° 

40.78° 

10.29° 

51.90° 

38.95° 

12.95° 

12  feet 

49  00° 

44.20° 

4  80° 

50  30° 

41  60° 

8  70° 

24  feet 

47  50° 

46  12° 

1  38° 

48  25° 

44  35° 

3  90° 

Soils  Due  Primarily  to  Disintegration. — Disintegration  explains 
why  the  soils  of  dry  regions  are  typically  "  sandy,"  that  is,  they  are 
composed  of  small  angular  pieces  of  broken  rock  rather  than  the  more 
or  less  rounded  grains  of  silica  which  compose  most  of  the  soils  of  humid 
regions.  In  other  words,  the  soils  of  dry  regions  are  comparatively 
1  Quoted  from  Forbes  by  Merrill,  Rocks,  Rock  Weathering  and  Soils. 


DISINTEGRATION  AND  ITS  PROCESSES 


75 


fresh;  they  are  largely  composed  of  small  pieces  of  rocks  which  have 
suffered  but  little  chemical  change.  On  the  other  hand,  the  soils  of 
humid  regions  are  derived  largely  from  rocks  which  have  been  decom- 
posed and  the  soils  have  been  leached  as  shown  on  page  71. 

The  peculiarities  of  arid  soils  are  shown  in  the  Fresno  loam,  of  which 
a  mineralogical  determination  is  given  in  the  following  table:1 


MINERALS  OTHER  THAN 
QUARTZ  IN 

ABUNDANT  AND  CHARACTERISTIC 
MINERALS  IN 

Remarks. 

Sand, 
per  cent 

Silt, 

per  cent 

Sand. 

Silt. 

30-50 

50-70 

Hornblende,  ortho- 
clase,  plagioclase 
feldspars. 

Biotite  horn- 
blende. 

Mineral  grains  distinctly 
angular  with  high  con- 
tent of  plagioclase  feld- 

spars. 

It  is  evident  first  that  these  soils  contain  a  high  content  of  minerals 
other  than  quartz.  Then  the  plagioclase  feldspars  which  weather  more 
easily  than  the  orthoclase  are  especially  abundant  and  in  a  fresh  con- 
dition. Finally  the  angularity  of  the  soil  grains  testifies  to  the  slight 
amount  of  solution  which  they  have  undergone. 

Exfoliation. — Rocks  are  poor  conductors  of  heat,  so  that  the  rock  a 
few  inches  below  an  exposed  surface  becomes  much  warmer  or  cooler 
than  the  interior.  As  a  result  of  the  differential  expansion  and  con- 
traction due  to  this,  a  crevice  starts  between  the  two  zones  of  consid- 
erable and  of  small  changes;  this  crevice  is  extended  with  successive 
temperature  changes  and  eventually  there  is  a  scaling  off  of  the  outer 
shell,  a  process  termed  exfoliation  or  spherical  weathering.  Exfoliation 
is  best  developed  in  massive  rocks,  those  having  much  the  same  structure 
throughout,  because,  if  there  are  many  differences  either  of  composi- 
tion or  structure,  the  layers  will  crumble  or  break  up  instead  of  scaling 
off  in  large,  thin  flakes.  Again,  exfoliation  is  best  shown  in  fairly  strong 
rocks ;  shales  and  other  weak  rocks  break  off  in  small  pieces  rather  than 
in  large  flakes.  The  process  is  effective  from  small  pebbles  to  large 
mountain  masses,  as  seen  in  Fig.  67. 

Other  Factors. — The  color  of  a  rock  has  an  obvious  effect  in  disin- 
tegration, for  it  is  a  well-known  fact  that  dark-colored  rocks  absorb 

1  The  Microscopic  Determination  of  Soil-forming  Minerals,  by  W.  J.  McCaughey 
and  William  H.  Fry,  U.  S.  Bureau  of  Soils,  Bulletin  No.  91,  1913. 


76  WEATHERING 

more  heat  than  light-colored  ones.  A  minor  but  persistent  factor  is 
the  beating  of  rain  drops,  the  effect  of  which  is  seen  on  incoherent  mate- 
rials like  mud,  but  it  also  acts  less  conspicuously  on  harder,  firmer  rocks. 
Freezing  is  an  important  disintegration  factor  over  large  areas  where 
the  temperatures  reach  the  freezing-point  of  water.  Water  in  solidifying 
expands  about  9  per  cent  of  its  bulk  with  a  force  of  150  tons  to  the  square 
foot,  a  pressure  which  will  easily  rend  strong  iron  pipes  or  perceptibly 
raise  a  building.  It  is  readily  apparent  that,  once  water  penetrates 
rock,  its  great  expansive  power  in  freezing  will  break  the  strongest  rock. 
Cracks,  joints,  bedding  planes  or  planes  of  schistosity  all  furnish  com- 
paratively easy  entry  for  water  and  also  are  lines  of  weakness  which 


FIG.  67. — "Enchanted  Rocks,"  Texas.     The  hills  are  of  granite  and  show  exfolia- 
tion on  a  large  scale.     (Paige,  U.  S.  Geological  Survey.) 

readily  yield  to  ice  expansion.  Water  also  enters  the  pores  of  rocks, 
but  here  the  ice  work  is  sometimes  not  so  effective  unless  all  the  pores 
are  filled,  since  the  ice  can  to  some  extent  expand  into  adjoining  vacant 
pore  spaces.  Like  many  other  weathering  processes,  the  rending  by  ice 
is  somewhat  cumulative;  each  freezing  leaves  a  crevice  somewhat 
larger  and  deeper  than  before  and  makes  succeeding  effects  easier. 

Freezing  and  Thawing. — This  process  is  especially  effective  in  regions 
of  repeated  freezing  and  thawing,  such  as  many  mountain  tops,  which 
are  often  so  covered  with  angular  debris  that  the  underlying  rocks  can- 
not be  seen.  On  some  mountains  one  can  often  note  with  considerable 
certainty  where  the  less  angular  debris  due  to  weathering  changes  to 
the  angular  debris  due  largely  to  disintegration  by  freezing.  Indeed, 
some  geologists  hold  that  the  active  weathering  due  to  disintegration 
on  mountain  tops  is  so  effective  as  to  limit  mountain  growth,  that  is, 


DISINTEGRATION  AND  ITS  PROCESSES 


77 


according  to  this  theory,  mountains  cannot  be  elevated  above  a  certain 
height  because  of  the  increasing  effectiveness  of  disintegration.  Cleo- 
patra's needle  is  often  mentioned  as  an  illustration  of  the  effectiveness  of 
freezing.  This  granite  obelisk  had  stood  for  centuries  without  injury 
in  the  warm,  equable  climate  of  Egypt,  but,  when  removed  to  New 
York,  the  inscriptions  soon  began  to  show  the  effects  of  repeated  freezing 
and  thawing,  and  it  was  found  necessary  to  coat  the  surface  with  par- 
affin to  keep  out  the  water. 

One  of  the  beneficial  effects  of  fall  plowing  is  due  to  freezing;  frag- 
ments of  rock  are  broken  and  the  plant  food  made  more  easily  available 
and,  furthermore,  the  heaving  of  the  soil  leaves  it  more  open  so  that  the 
tilth  is  improved.  This  break- 
ing up  of  soil  particles  is  well 
illustrated  by  feldspar  pebbles 
found  in  the  western  High 
Plains.  These  pebbles  have 
been  rounded  by  water  action, 
but  freezing  water  acting  along 
the  lines  of  easy  cleavage  in 
the  pebbles  has  broken  many 
of  them  into  somewhat  angu- 
lar, smaller  shapes.  The 
farmer  in  cool  regions  is  often 
puzzled  by  the  recurrence  each 
spring  of  a  new  crop  of 
stones  to  be  picked.  The 
explanation  lies  in  the  fact 
that,  as  the  water  in  the  soil 

freezes,  the  rocks  are  pushed  upward  by  the  expansion  and  do  not  sink 
back  so  far  as  the  finer  materials  when  the  soil  thaws  and,  as  a  result, 
the  rocks  and  pebbles  are  left  nearer  the  surface  which  they  finally  reach. 

Gravity  is  an  important  weathering  factor  on  slopes,  its  importance 
increasing  with  the  steepness  of  the  slopes.  On  all  slopes  there  is  a 
movement  down  slope  of  the  mantle  rock,  a  movement  usually  imper- 
ceptibly but  fairly  continuous.  Soils  and  mantle  rock  are  thereby 
carried  down  slope,  leaving  the  rocks  on  upper  slopes  less  protected  and 
thereby  the  more  exposed  to  weathering. 

REFERENCES 

E.  W.  HILGARD,  Soils,  Chapter  1,  Disintegration. 

G.  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  1906,  Part  3,  pages  158-163; 
175-180, 


FIG.  68. — Residual  boulders  surrounded  by 
soft,  disintegrated  granite.  (Weed,  U.  S. 
Geological  Survey.) 


78  WEATHERING 

The  Weathering  Work  of  Plants 

The  weathering  work  of  plants  is  materially  conditioned  by  climate 
and  soil.  In  the  moist,  hot  tropics  where  plants  grow  in  profusion, 
this  work  is  very  important,  while  in  cold  or  arid  regions  or  on  rocky 
slopes  the  work  of  plants  may  be  of  slight  importance.  On  the  whole 
plants  favor  decomposition  rather  than  disintegration,  but  a  heavy 
growth  of  plants  often  in  some  measure  protects  from  weathering. 

The  Work  of  Roots. — The  prying  effect  of  roots  is  a  common  occur- 
rence. A  root,  starting  in  a  crevice,  joint  or  some  plane  of  weakness, 
extends  inward  and  at  the  same  time  increases  in  diameter  and  so  widens 
the  opening.  Roots  in  their  successive  descent  push  the  soils  back  and 
.forth  and  so  render  them  more  open,  an  effect  well  shown  where  trees 
are  set  out  in  a  stiff  hard  pan  and  the  roots  finally  break  up  the  hard 
pan.  In  Florida,  for  example,  pineapples  are  set  out  in  a  friable  lime- 
stone which  the  roots  penetrate  and  break  up.  The  roots  of  grains 
penetrate  to  a  considerable  depth,  and  alfalfa  roots  have  been  found  at 
depths  of  30  or  more  feet  and  the  intricate  tangle  of  corn  roots  some- 
times penetrates  to  depths  of  6  feet  or  more  and  they  extend  for  vary- 
ing distances  from  the  plant.  The  overturning  of  trees  stirs  the  soil 
to  a  considerable  depth  and  brings  relatively  fresh  soil  to  the  surface, 
there  to  be  exposed  to  active  weathering. 

Roots  have  a  solvent  action,  as  is  shown  by  the  familiar  experiment 
of  growing  plants  on  a  polished  marble  surface  where  the  root  paths 
may  be  traced  by  faintly  etched  lines  of  roughened  surface.  Roots, 
tree  roots  especially,  when  they  decay  provide  paths  for  descending 
water.  A  bare  rock  surface  in  a  humid  climate  is  soon  covered  with  a 
growth  of  mosses  and  lichens  which  keep  the  surface  damp,  the  rootlets 
penetrate  crevices  and  the  decaying  plants  furnish  acids  which  attack 
the  rock,  all  of  which  convert  the  rock  into  soil  and  prepare  it  for  a 
growth  of  higher  plants.  Plant  roots  bring  to  the  upper  soil  some  of  the 
soluble  compounds  from  below  and  so  promote  an  interchange  of  mate- 
rials. From  an  agricultural  point  of  view  this  bringing  up  of  compounds 
of  potash  and  phosphoric  acid  is  of  considerable  importance. 

Decay  and  humification  are  extremely  important  in  weathering 
work  of  plants.  When  leaves,  stems  and  trunks  fall  to  the  surface  of 
the  ground  and  are  freely  exposed  to  the  air,  they  change  for  the  most 
part  to  carbon  dioxide  (662)  and  water.  A  portion  of  the  carbon  diox- 
ide is  dissolved  in  the  rain  water,  carried  into  the  soil  and  performs 
the  work  which  has  been  described  in  preceding  pages.  On  the  other 


THE  WEATHERING  WORK  OF  PLANTS  79 

hand,  when  vegetable  matter  falls  into  water  or  on  moist  surfaces, 
there  is  less  access  of  oxygen  and  complete  decay  does  not  occur.  Roots 
and  other  parts  of  plants  under  many  conditions  are  surrounded  by  soil 
so  moist  much  of  the  time  that  they  do  not  completely  decay.  The 
organic  matter  resulting  from  such  incomplete  decay  is  termed  humus. 
It  is  the  substance  that  gives  the  dark  color  to  so  many  productive 
soils  so  that  a  dark  soil  is  commonly  regarded  as  productive  and  modern 
agricultural  methods  emphasize  the  creation  of  humus  if  it  is  not  already 
present.  The  term  in  agricultural  literature  is  commonly  used  some- 
what indefinitely,  sometimes  meaning  simply  decomposed  vegetable 
matter  and,  again,  vegetable  matter  so  decomposed  and  changed  into  a 
black  waxy  substance  that  nearly  all  traces  of  plant  remains  have 
disappeared. 

Humus  is  abundant  in  favorable  locations  and  scarce  in  others.  In 
arid  soils  it  forms  very  slowly  if  at  all.  Hilgard  found  the  average 
humus  percentage  in  arid  uplands  to  be  only  0.91  per  cent  and  in  soils  of 
humid  climates,  4.58  per  cent.  Moisture  favors  the  growth  of  vege- 
tation and,  under  favorable  conditions,  protects  the  vegetable  matter 
from  the  air,  thus  preventing  a  rapid  oxidation  so  that  humus  is  notably 
developed  in  two  localities,  swamps  and  humid  prairies.  In  swamps  it 
accumulates  and  often  forms  peat.  On  prairies  in  the  humid  regions 
there  is  commonly  a  deep  layer  of  humus.  The  black  soil  in  the  Iowa 
corn  belt  is  often  6  feet  deep  and  the  Marshall  silt  loam,  a  very  pro- 
ductive prairie  soil  of  loessial  origin,  has  a  deep  layer  of  soil  mingled 
with  humus.  The  distinctive  feature  of  prairies  which  favors  the 
accumulation  of  humus  is  their  level  or  gently  rolling  surface,  which 
prevents  too  rapid  drainage,  keeps  the  soil  fairly  moist  and  so  favors 
the  slow  decomposition  which  results  in  humus.  There  seems  to  be  a 
difference  not  well  understood  between  the  humus  of  prairies  and  that 
of  swamps;  the  former  will  endure  long  cultivation  while  the  latter  is 
rapidly  oxidized  and  disappears  with  a  few  years  of  cultivation.  The 
large  amount  of  humus  in  prairie  and  other  soils  is  doubtless  due  in 
large  measure  to  the  decay  of  roots  since  a  close  correspondence  has 
been  noted  between  root  growth  and  the  distribution  of  humus.  Other 
things  being  equal,  soil  texture  has  a  considerable  effect  on  the  forma- 
tion of  humus.  The  lighter  soils  such  as  sandy  loams  or  sands  allow 
free  access  of  air  by  which  vegetable  matter  is  rapidly  oxidized  and, 
moreover,  they  favor  rapid  soil  drainage  so  that  the  soil  is  not  moist 
for  long  enough  periods  to  favor  the  accumulation  of  humus.  The 
Houston  soil  series  in  Texas  affords  a  good  example  of  the  effects  of  soil 


80  WEATHERING 

texture.  The  usual  soils  in  this  series  are  heavy  but  small  areas  of 
sandy  loam  are  frequent.  The  clay  loams  of  this  series  contain  6.31 
per  cent  of  vegetable  matter  while  the  fine  sandy  loams  contain  only 
2.38  per  cent. 

Weathering  Effects  of  Humus. — Humus  is  so  important  agricul- 
turally that  its  formation  has  been  considered  in  some  detail,  but  it 
should  not  be  forgotten  that  it  is  an  agent  of  no  small  importance  in 
soil  weathering.  Because  of  its  porosity  humus  absorbs  and  retains 
moisture  which  is  effective  in  soil  solution  work.  The  absorbtive  qual- 
ities* of  humus  will  be  apparent  from  the  following  table  compiled  by 
Lyon  and  Fippen.1 

PER  CENT  OF  WATER  IN  SOILS  AT  SATURATION 

Coarse  sand 40 . 5  per  cent 

Fine  sandy  loam 38 . 0  per  cent 

Clay 54 . 5  per  cent 

Humus .  . 333 .0  per  cent 

Humus  expands  with  moisture  and  contracts  upon  drying  and 
these  movements,  together  with  the  porosity  of  humus  itself,  tends  to 
keep  soils,  especially  heavy  soils,  open  for  the  movements  of  air  and 
moisture.  Humic  acids,  mainly  carbonic  acid,  are  generated  when 
humus  is  oxidized.  Thus  humus  has  important  weathering  effects, 
both  chemically  and  physically. 

REFERENCE 

LYON,  FIPPEN  AND  BUCKMAN,  Soils,  their  Properties  and  Management,  Macmillan, 
1915,  Chapter  8,  The  Organic  Matter  of  the  Soils. 

Microorganisms,  mainly  bacteria  and  fungi,  break  down  plant  tis- 
sues and  so  promote  the  formation  of  humus.  These  organisms  are 
sometimes  found  in  enormous  numbers;  Mayo  and  Kinsley  found  a 
black  loam  in  Kansas  which  contained  over  thirty  million  bacteria  per 
cubic  centimeter,  but  not  all  these  organisms  are  active  in  the  produc- 
tion of  humus.  The  work  of  these  organisms  is  practically  confined  to 
a  shallow  soil  zone  beneath  the  surface.  Acids  in  the  soil  or  subsoil 
retard  the  growth  and  activity  of  microorganisms;  if  a  soil  is  cal- 
careous the  acids  are  neutralized  and  the  soil  is  more  favorable  to  the 
action  of  these  small  forms.  This  is  one  of  the  reasons  why  so  many 
limestone  soils  are  high  in  humus  and  dark  in  color.  For  example,  the 
1  Soils,  Lyon  and  Fippen,  New  York,  1910,  page  K>1. 


THE  WEATHERING  WORK  OF  ANIMALS 


81 


soils  of  the  Houston  series  are  derived  from  a  clayey,  "  rotten  "  lime- 
stone which  weathers  for  the  most  part  to  heavy,  calcareous  soils  which, 
because  of  their  color,  give 
their    name    to    the    pro- 
ductive  "Black   Belt"   of 
Alabama  and  Mississippi. 

Bacteria. — The  enorm- 
ous numbers  of  these  small 
plants  and  their  impor- 
tance on  the  formation  of 
humus  in  some  soils  have 
been  noted.  The  work  of 
many  of.  these  bacteria  is 
but  little  understood  but 
some  groups  are  of  in- 
terest from  a  weathering 
standpoint.  One  group, 
the  nitrifying  bacteria,  con- 
verts ammonia  compounds 
into  nitric  acid  and  ni- 
trates while  others  con- 
vert nitrates  into  simpler 
compounds.  The  root  bac- 
teria, Fig.  69,  of  legumes 


FIG.  69. — " Lumps"  of  root  bacteria  growing  on 
alfalfa  roots.     (C.  W.  Edgerton.) 


such    as  peas,   clover    and 

alfalfa  obtain  nitrogen  from 

the   air  and  furnish  it  to 

the  plants  from  which  some  nitrogen  in  some  form  remains  in  the 

soil. 


[REFERENCES 

H.  W.  CONN,  Agricultural  Bacteriology,  Part  2,  Blakiston,  1918,  pages  41-137. 
J.  G.  LIPMAN,  Bacteria  in  Relation  to  Country  Life,  Macmillan,  1908,  Part  4,  pages 
135-302,  Bacteria  in  Relation  to  Soil  Fertility. 


The  Weathering  Work  of  Animals 

The  work  of  animals  in  promoting  weathering  is  mostly  performed 
by  those  of  burrowing  habits.  Such  work,  therefore,  is  largely  confined 
to  rock  that  is  already  broken  up,  and  since  animals  do  not  burrow  far 


82  WEATHERING 

below  the  surface,  their  work  is  for  the  most  part  confined  to  the  soil 
zone.  These  animals  bring  up  lower  soil  to  be  acted  on  more  or  less  vig- 
orously by  weathering  agents  and  their  burrows  allow  easy  entry  of 
air  and  water.  The  geologist  and  soil  surveyor  is  often  helped  in  his 
work  by  observing  the  deeper  materials  thrown  out  by  burrowing 
animals. 

In  humid  climates  probably  the  most  important  animal  from  a 
weathering  standpoint  is  the  earthworm.  Darwin,  in  his  classical 
researches,  estimated  that  in  some  soils  there  are  50,000  earthworms  to 
the  acre  and  that  in  half  a  century  these  earthworms  will  work  over 
from  one-half  to  an  entire  surface  foot  of  soil.  They  pass  the  soil  through 
their  bodies  to  get  its  vegetable  matter  and  so  break  up  and  decompose 
the  soil  particles.  In  poorly  drained  areas  the  crawfish  does  consider- 
able weathering  work  as  a  burrowing  animal.  The  term  "  crawfish 
land  "  indicates  the  activity  of  these  animals  and  the  accompanying 
poor  drainage.  The  work  of  ants  is  important  in  the  tropics  but  of 
much  less  importance  in  temperate  climates.  Ants  bring  soil  to  the 
surface  and  their  connecting  underground  passages  extending  for  miles 
afford  opportunity  for  the  entrance  of  air  and  water.  They  carry 
underground  food  such  as  leaves  and  other  vegetable  matter  which,  in 
decaying,  yields  carbon  dioxide. 

REFERENCES 

N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Report,  Part  1,  U.  S.  Geological 

Survey,  Animals  and  Plants  in  Soils,  pages  268-287. 
VAN  HISE,  A  Treatise  on  Metamorphism,  Weathering  Due  to  Plants  and  Animals. 

Decomposition,  pages  452-457;   Disintegration,  pages  444-451. 

Interaction  of  Weathering  Factors 

It  should  be  kept  in  mind  that  weathering  processes,  both  chemical 
and  physical,  not  only  go  on  together  but  are  mutually  helpful.  It  is 
seldom  that  every  process  is  equally  effective  at  a  given  time  and  place, 
but  one  or  another  process  is  usually  dominant  and,  where  all  processes 
operate  actively  together,  weathering  goes  on  very  actively..  It  has 
been  noted  that  disintegration  is  dominant  in  arid  regions  while  decom- 
position is  very  prominent  in  humid  regions.  Disintegration  by  expan- 
sion and  contraction  and  by  freezing  breaks  up  rock  and  so  paves  the 
way  for  more  effective  action  by  the  agents  of  decomposition.  For 
example,  a  cube  of  rock  one  inch  square  exposes  six  square  inches  to 


RATE  OP  WEATHERING  83 

weathering;  if  this  cube  be  broken  into  four  equal  cubes,  there  will  be 
a  much  larger  area  exposed.  Again,  if  this  cubic  foot  be  comminuted 
to  the  texture  of  ordinary  loam,  there  is  a  total  surface  according  to 
King  of  about  an  acre  and,  according  to  the  same  author,  in  a  fine  clay 
there  is  as  much  as  four  acres  of  exposed  surface.  When  it  is  remem- 
bered that,  in  humid  regions,  the  small  soil  grains  are  often  surrounded 
by  films  of  capillary  water  and  that  this  water  is  decomposing  the  soil 
grains,  it  is  at  once  clear  that  the  breaking  up  of  rocks  and  soils  into 
very  fine  particles  is  an  important  aid  to  weathering.  It  is  also  evident 
in  this  connection  that  the  rapid  increase  in  water-holding  capacity 
as  soil  particles  become  smaller  is  an  important  weathering  factor. 

The  figures  given  above  are  only  estimates,  but  they  will  give  an 
idea  of  the  great  aid  which  disintegration  renders  to  decomposition  by 
breaking  up  the  rocks  and  soil  particles.  The  pores,  crevices  and  other 
spaces  made  by  the  processes  of  disintegration  facilitate  the  entrance 
of  air  and  water  which  attack  the  minerals.  Conversely  it  has  been 
seen  that  oxidation,  carbonation  and  hydration  are  accompanied  by 
increases  in  bulk  which  tend  to  disrupt  the  rocks  and  to  further  assist 
disintegration.  It  has  been  calculated  that,  in  the  change  from  granite 
to  soil,  there  is  an  increase  in  bulk  of  88  per  cent  provided  there  is  no 
loss  of  materials,  but  this  statement  is  useful  mainly  for  illustration  since 
there  is  always  some  loss  of  materials;  the  statement  will,  however,  show 
the  potency  of  an  increase  in  bulk  in  hastening  weathering  processes. 
Again,  it  should  be  remembered  that  hydration,  oxidation  and  carbona- 
tion very  seldom  operate  separately  but  work' simultaneously.  Espe- 
cially are  oxidation  and  carbonation  commonly  associated  but,  while 
oxidation  and  carbonation  are  mainly  limited  to  a  relatively  shallow 
zone,  hydration  can  and  does  occur  whenever  water  is  present  in  the 
rocks,  even  to  great  depths.  Probably  carbonation  is  the  most  vari- 
able process  since,  while  carbon  dioxide  is  universally  present  in  the  air, 
its  content  in  the  ground  water  is  variable.  The  weathering  products 
of  minerals  are  nearly  always  much  weaker  than  the  original  minerals. 
Hornblende  and  feldspars,  for  example,  weather  to  incoherent  quartz 
and  elays. 

Rate  of  Weathering 

Probably  the  most  complete  and  rapid  weathering  is  to  be  found  in 
the  humid  tropics  where  high  temperatures,  much  moisture  and  abun- 
dance of  vegetation  combine  to  provide  verj  favorable  conditions  for 


84  WEATHERING 

decomposition.  Leaching  is  very  complete  and  residual  products  are 
rich  in  hydrated  minerals.  Here  is  found  a  mantle  rock  to  which  the 
term  laterite  is  applied.  It  is  a  reddish  material  in  which  the  parent  rocks 
seem  to  be  entirely  decomposed,  even  durable  silicates  being  completely 
broken  up  and  changed.  Rocks  under  favorable  conditions  in  the  tropics 
have  been  weathered  to  depths  of  two  hundred  feet  or  more.  In  arid 
regions,  both  hot  and  temperate,  the  conditions  are  highly  favorable 
for  disintegration  since  there  is  little  moisture  for  decomposition.  The 
dry  air  is  conducive  to  great  and  sudden  temperature  changes  which 
produce  marked  expansion  and  contraction  and  freezing  and  thawing 
are  active  in  deserts  of  the  temperate  zones. 

Altitude  on  the  whole  favors  disintegration.  As  altitude  increases 
the  cold  becomes  greater  and  daily  and  seasonal  variations  are  more 
marked  and  when  the  altitude  is  sufficient  for  freezing  this  factor  is 
added  to  the  forces  of  disintegration.  Furthermore, ,  high  altitudes  are 
likely  to  be  characterized  by  more  or  less  steep  slopes  which  add  to  the 
rapidity  of  weathering.  The  greatest  effect  of  freezing  is  found  in 
the  temperate  zones  where  alternate  freezing  and  thawing  repeatedly 
occur  during  a  single  winter.  Here  also  are  found  the  much  less  impor- 
tant temperature  variations  between  summer  and  winter  which  are 
effective  to  greater  depths  than  are  daily  variations.  In  the  polar 
regions  the  temperature  variations  and  freezing  and  thawing  are  of 
small  importance  since,  excepting  a  thin  veneer  of  soil,  the  ground  is 
frozen  to  a  considerable  depth  all  the  year  around.  The  body  of  a 
mammoth,  an  animal  extinct  for  thousands  of  years,  was  found  frozen 
in  Siberia,  a  fact  that  indicates  that  the  ice  in  that  region  has  been  pre- 
served since  early  Pleistocene  times.  A  minor  effect,  but  very  inter- 
esting for  local  study,  is  the  contrast  in  weathering  on  north  and  south 
slopes.  In  the  Northern  Hemisphere  snow  and  freezing  persist  longer 
on  northern  slopes  but  vegetation  is  usually  heavier  on  southern  slopes. 
It  has  been  observed  in  the  mountains  of  the  Carolinas  that,  other  things 
being  equal,  the  soils  on  northern  slopes  are  likely  to  be  darker  in  color 
than  those  on  southern  slopes.  In  other  words,  decomposition  is 
less  active  on  the  northern  slopes  and  the  soils  are  likely  to  contain 
more  humus. 

REFERENCES 

VAN  HISE,  A  Treatise  on  Metamorphism,  Rate  of  Weathering,  pages  532-534. 
RIES  AND  WATSON,  Engineering  Geology,  Wiley  &  Sons,  1914,  Chapter  4   (general 
treatment). 


CHAPTER    IV 
RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 

LIMESTONE  AND   MARBLE  SOILS 

Introductory. — Limestone  soils  are  proverbially  fertile  and  of  large 
extent.  It  will  be  remembered  that  the  basic  mineral  in  limestone  and 
marble  is  calcite  (CaCOs),  which  is  readily  soluble  in  water.  However, 
limestone  in  nature  is  practically  never  pure.  Because  of  its  mode  of 
formation,  there  are  almost  always  varying  amounts  of  clay  and  sand, 
so  that  limestones  grade  into  shales  through  the  argillaceous  (clayey) 
limestones  and  into  sandstones  through  the  aranaceous  (sandy)  lime- 
stones. Clayey  limestones  typically  yield  heavy  soils  such  as  clays  and 
silts.  Sandy  limestones  often  yield  loams  and  silt  loams.  A  very 
common  impurity  is  silica  (SiOa)  in  the  form  of  flint  or  chert,  which  may 
occur  in  small  grains  and  is  often  distributed  in  nodules  or  layers  as 
shown  in  Fig.  36.  The  chert  and  flint  dissolve  very  slowly  as  com- 
pared with  the  enclosing  limestone  and  are  left  in  the  residual  soil 
as  gravel  or  small  rocks,  thus  often  forming  a  stony  or  gravelly  loam. 
For  example,  the  widespread  Clarksville  series  of  soils  is  derived  from  a 
somewhat  siliceous  limestone  and  the  soils  are  often  somewhat  stony 
and  cherty. 

A  limestone  soil  represents  the  residuum  of  a  large  mass  of  rock 
that  has  been  carried  away  in  solution.  Pumpelly,  in  describing  the 
weathering  of  limestones  in  the  Missouri  Ozarks,  estimates  that  the  re- 
sidual insoluble  materials  from  20  to  120  feet  in  depth  have  been  derived 
from  not  less  than  1200  feet  of  limestones.  In  southern  Wisconsin 
there  are  residual  clays  with  an  average  depth  of  about  10  feet  and 
Whitney  estimates  that  these  were  derived  from  the  decomposition  of 
from  350  to  400  feet  of  formerly  overlying  limestone  and  calcareous 
shales.  These  estimates  are  little  more  than  suggestions  and  are  prob- 
ably underestimated  since  much  of  the  residual  material  has  been 
washed  away, 

85 


86  RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 

Clay  Soils  from  Limestone. — The  principal  impurity  of  limestone  is 
clay,  which,  being  practically  insoluble,  remains  after  the  calcite  has 
been  dissolved  and  carried  away;  it  is,  therefore,  clear  that  limestone 
soils  are  typically  clays  and  silts.  Limestone  soils  are  proverbially 
fertile  although,  contrary  to  a  common  impression,  they  do  not  neces- 
sarily contain  much  lime  since  the  lime  carbonate  is  more  or  less  leached 
out.  Indeed  a  common  practice  in  many  limestone  regions  is  to  lime 
the  soils  somewhat  frequently  to  correct  soil  acidity.  Limestone  soils 


FIG.  70. — Limestone  and  its  residual  soil.  The  rock  which  yielded  the  soil  was 
many  times  thicker  than  the  present  zone  of  soil  and  mantle  rock.  (U.  S.  Geo- 
logical Survey.) 

are  often  reddish  either  in  the  soil  or  subsoil  unless  they  contain  enough 
humus  to  make  them  dark.  The  reddish  colors  are  due  to  finely  divided 
oxides  of  iron  which  coat  the  clay  and  silt  particles,  the  colors  being  a 
result  of  the  long-continued  oxidation  which  the  soils  have  undergone. 
Soils  from  Cherty  Limestones. — Chert  weathers  slowly;  when  it  is 
pure  the  weathering  is  almost  wholly  by  disintegration — freezing  and 
thawing  and  contraction  and  expansion  being  especially  prominent. 
Most  cherts,  however,  have  small  amounts  of  calcite  scattered  through- 
out and  these  are  dissolved  out  leaving  the  chert  pitted  and  porous. 


LIMESTONE  AND  MARBLE  SOILS  87 

Furthermore,  cherts  are  very  slowly  soluble  in  alkaline  water  which  is 
commonly  found  in  limestone;  under  weathering  they  often  change 
from  hard,  firm  chert  to  a  soft,  porous  "  cotton  rock  "  which  can  be 
picked  apart  with  the  fingers.  Hence  it  is  that  the  slowly  weathering 
cherts  and  flints  remain  in  limestone  soils  long  after  the  limestone  has 
been  dissolved  and  soils  from  cherty  limestones  are  often  stony  or 
gravelly.  If  weathering  has  been  in  progress  for  a  long  time,  the  cherts 
are  often  broken  into  fine  particles  and  the  soils  are  somewhat  sandy  in 
consequence. 

Soils  from  Dolomitic  Limestones. — Dolomitic  (magnesian)  lime- 
stones are  widespread.  These  limestones  weather  more  slowly  than 
ordinary  limestone  and,  where  the  two  are  in  the  same  locality,  the 

SILT  LOAMS, 

GRAVELLY  AND     LOAMS  AND     GRAVELLY  L°AMS    GRAVELLY  AND 
STONY  LOAMS      SILT  LOAMS      LOAMS   CLAYS    STONY  LOAMS 


FIG.  71. — Diagram  to  illustrate  the  topography  and  soils  from  cherty  dolomitic 
limestones,  limestones  and  sandstones  and  shales  in  northern  Georgia. 

(KD),  Knox  dolomite;     (CS),  limestones;    (CL)  sandstones  and  shales.      (Data  from  U.   S. 
Bureau  of  Soils  and  U.  S.  Geological  Survey.) 

dolomitic  limestones  usually  make  the  higher  topography.  On  a  small 
scale  this  differential  weathering  is  often  shown  where  the  dolomitic 
portions  of  a  limestone  weather  slowly  and  stand  in  relief.  This  is 
especially  well  shown  in  the  Appalachian  Ridges  of  Tennessee,  Georgia 
and  Alabama  where  the  Knox  dolomite,  an  important  soil-making  forma- 
tion, underlies  most  of  the  ridges,  the  valleys  being  underlain  by  weaker 
limestones  and  shales.  The  formations  in  this  belt  have  been  folded 
so  that  they  are  repeatedly  brought  to  the  surface  as  shown  in  Fig.  71. 
The  residual  soils  illustrate  many  of  the  preceding  principles  and  are, 
therefore,  worth  a  somewhat  detailed  consideration. 

A  somewhat  typical  illustration  is  shown  in  Fig.  71.  The  Knox  dolomite  (KD) 
here  is  a  somewhat  massive  magnesian  limestone  which  contains  considerable  flint. 
Both  because  of  its  silicious  composition  and  its  slowly  soluble  magnesian  carbon- 


88 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


ate,  this  rock  has  been  less  eroded  than  purer  limestones  and  shales  and  it  stands 
up  as  roughly  parallel  ridges.  The  residual  soils  of  the  different  rocks  show  inter- 
esting variations.  The  Knox  dolomite  (KD)  yields  a  slit  loam,  but  this  formation 

contains  much  silica  in  the*  form  of  flint  and 
chert  arranged  both  as  nodules  and  as  thin 
layers.  The  silica  weathers  very  slowly  so 
that  chert  of  various  sizes  and  angular  shapes 
is  scattered  through  the  soil  and  subsoil. 
Layers  of  partly  decomposed  chert  are  found 
in  their  original  position  in  the  subsoil.  The 
soil  is  a  gravelly  or  a  stony  loam  and  the 
surface  contains  much  more  stony  matter 
than  the  subsoil  because  the  finer  particles 
have  been  carried  away,  leaving  the  coarser 
chert  fragments  behind.  The  Chickamauga 
limestone  (CL)  varies  from  fairly  pure  lime- 
stone to  clayey  limestone.  From  the  former 
is  derived  a  deep  silt  loam  and  from  the  latter, 
a  clay  soil.  The  more  soluble^  lowland^ 
has  weathered  to  a  gently  rolling  limestone ^ 

A  silt  loam  is  derived  from  the  Conasauga  shale  (CS),  a  formation  mostly  of  shale 
but  with  interbedded  limestone  and  limey  shale.  Local  areas  of  fairly  pure  lime- 
stone yield  clay  soils  and  cherty  areas  yield  loams.  In  places  the  original  strati- 
fication of  the  shales  is  still  preserved  in  some  of  the  subsoils. 

Chemical  and  Mineralogical  Changes. — The  changes  in  composition 
from  a  fresh  limestone  to  its  residual  clay  are  illustrated  in  Fig.  73. 


FIG.  72. — Weathering  of  cherty 
dolomitic  limestone.  The  white 
portions  (chert)  are  less  weath- 
ered and  project.  (Mo.  Geologi- 
cal Survey.) 


1 

SILICA                            7.41$ 
ALUMINA                       1.91$ 
IRON    OXIDE                0.98$ 
LIME                             28.29$ 
MAGNESIA                   18.17$ 
POTASH                         1.08$ 
CARDON  DIOXIDE     4.1.57$ 
WATER                          0.  57$ 

FRESH   LIMESTONE 
)                          25$                    5f; 
I 

;( 

RESID 
)                          2 

UAL  CLAY 
5*                         5 

0$ 

57.  5T$ 
20.44$ 
7.93$ 
0.51$ 
1.21$ 
4.91$ 
0.38$ 
6.69$ 

j 

CSSHBBB 

gjjgj 

r  -i  '    i  '  i  '-T-'-I- 

=| 

,   t    ,   <    ,    j 

3 

H 

1  i  '  i-j  i  '  i  ' 

-i-rJ-r-M 

ggg 

FIG.  73. — Diagram  showing  the  compositions  of  fresh  magnesian  limestone  and  its 
residual  clay.     (Data  after  Merrill.) 

The  fresh  rock  is  a  magnesium  limestone  composed  largely  of  lime  and 
magnesium  carbonates  with  some  silica  and  a  little  potash  and  iron. 
The  residual  clay  is  of  a  deep-red  color  as  indicated  by  the  considerable 
percentage  of  iron  oxides.  The  great  losses  involved  in  the  change  are, 
of  course,  the  soluble  carbonates  of  lime  and  magnesia,  while  the  great 


LIMESTONE  AND  MARBLE  SOILS 


relative  gains  in  the  residual  soils  are  in  the  silica,  alumina  and  water 
which  go  to  make  up  the  residual  clays.  The  iron  has  dissolved  much 
less  rapidly  and  shows,  therefore,  a  relative  increase  and  the  percentage 
of  potash  in  the  clay  is  largely  due  to  potash  feldspar  in  the  original 
limestone,  for  the  feldspars  have  not  completely  dissolved  but  remain 
in  the  residual  clays. 

The  following  mineralogical  analysis  of  a  limestone  soil  (Hagerstown 
series)  throws  some  light  on  the  processes  involved  in  the  origin.1 


MINERALS  OTHER  THAN  QUARTZ  IN 

ABUNDANT  AND  CHARACTERISTIC  MINERALS  IN 

Sand, 
per  cent. 

Silt, 
per  cent. 

Sand. 

Silt. 

5-8 

8-10 

Secondary  quarts  crystals, 
weathered  orthoclase. 

Altered  feldspar. 

In  the  first  place  there  is  a  total  absence  of  recognizable  calcite  in 
the  sands  and  silts.  The  clays  are  not  adapted  to  microscopic  study  and, 
therefore,  they  do  not  appear  in  these  tables.  By  secondary  quartz  is 
meant  quartz  that  has  been  deposited  in  the  limestone  ^ince  it  was 
formed.  These  tiny  quartz  particles  are  often  crystallized  and  show  the 
characteristic  six-sided  pyramids  so  familiar  in  large  crystals.  It  is 
clear  that  solution  is  eminently  active  in  the  weathering  of  limestone, 
at  least  in  humid  regions.  If  a  limestone  is  coarse-grained  there  is 
some  disintegration,  but  this  process  is  relatively  less  important  in  lime- 
stones than  other  rocks. 

Topography. — Another  soil  factor  in  limestone  regions  is  the  topog- 
raphy. Owing  to  the  easy  weathering  of  limestone  the  surface  is  often 
low  and  level  or  rolling.  There  is,  for  example,  a  stretch  of  level,  rolling 
limestone  country  along  the  Great  Valley  which  extends  from  New 
Jersey  to  Alabama.  The  Black  Belt  in  Alabama  and  Mississippi  and 
the  Black  Prairie  in  Texas  are  prairie  like  belts  underlain  by  weak 
("  rotten  ")  limestone  and  calcareous  shales.  The  relatively  leve1  sur- 
face, as  we  have  seen  (page  79),  favors  the  accumulation  of  humus, 
hence  the  local  names.  The  dolomite  ridges  of  the  Appalachian  Ridge 
Belt,  Fig.  71,  illustrate  another  relation  between  topography  and  lime- 
stone soils.  The  rocks  of  these  ridges,  being  more  resistant  than  the 
neighboring  limestones,  form  ridges  which  are  elevated  and  have 

1  McCaughey  and  Fry,  loc  cit. 


90  RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 

rather  steep  slopes.     As  a  result,  the  soils  are  prevailingly  stony  because 
most  of  the  finer  soil  particles  are  washed  %  a  way. 

Notable  Regions. — Limestones  are  widely  distributed  both  in  large 
and  in  small  areas.  The  limestone  belts  in  the  South  have  been  noted 
in  the  preceding  paragraph  (see  Fig.  251).  Long  limestone  valleys 
together  with  the  great  Appalachian  Valley  in  the  Appalachian  Ridge 
Belt  afford  limestone  soils  (Figs.  49  and  55).  The  Blue  Grass  Badns  in 


FIG.  74. — Soils  derived  from  limestone  in  the  foreground.  The  ridge  in  the  back- 
ground is. underlain  by  sandstone  and  is  covered  by  a  stony  loam.  (U.  S. 
Bureau  of  Soils.) 

Kentucky  and  Tennessee  are  floored  with  limestone  soils  (Fig.  59)  and 
there  are  large  areas  in  the  Ozarks  of  Missouri  and  Arkansas. 

REFERENCE 

H.  H.  BENNETT,  Soils  of  the  Limestone  Valleys  and  Uplands  Province  in  Soils  of  the 
United  States,  Bull.  96,  U.  S.  Bureau  of  Soils,  1913;  general,  pages  85-89,  Soil 
Series,  pages  89-108. 


SANDSTONE  AND   QUARTZITE  SOILS 

SANDSTONE  SOILS 

Introductory. — In  general  it  will  be  remembered  that  sandstones  are 
predominantly  composed  of  silica  and  that  they  themselves  are  products 
of  long-continued  weathering.  For  example,  granite  after  long-con- 
tinued weathering  changes  mainly  into  quartz  and  clay ;  the  quartz  and 
clay  are  usually  separated  by  stream  action,  the  quartz  forming  sand- 
stone and  the  clays  shales.  The  quartz  grains  forming  the  bulk  of 


SANDSTONE  AND  QUARTZITE  SOILS  91 

most  sandstones  are,  therefore,  very  resistant  to  weathering.  Nearly 
all  sandstones  contain  a  considerable  proportion  of  silt  and  clay  so  that 
the  soils  derived  from  apparently  coarse,  pure  sandstone  are  often 
surprisingly  heavy,  often  being/  loams  or  heavy  sandy  loams  rather 
than  sands.  Other  sandstones  are  calcareous,  but  calcareous  sandstones 
are  much  rarer  than  calcareous  shales  and  there  are  not  so  frequent 
gradations  between  sandstones  and  limestones  as  between  sandstones 
and  shales. 

There  is  an  important  soil  relation  between  the  size  of  grains  in 
the  parent  sandstone  and  the  texture  of  the  residual  soil.  Obviously  a 
coarse-grained  sandstone  will  yield  a  coarse-textured  sandy  soil  and  a 
fine-grained  sandstone  a  fine-grained  soil.  Furthermore,  a  fine-grained 
sandstone  is  deposited  by  relatively  slow  currents  and  is,  therefore, 
likely  to  contain  considerable  fine  materials  as  clay,  silt  and  fine  sand. 
Hence  it  is  that  fine-grained  sandstones  and  sandy  shales  often  yield 
silt  loams  and  very  frequently  they  yield  loams. 

The  weathering  of  sandstones  is  simple  and  consists  mainly -in  the 
solution  of  the  cements.  As  soon  as  the  cement  is  dissolved  the  grains 
fall  apart  and  the  sandstone  crumbles  into  soils.  Calcareous  cements 
dissolve  easily,  iron  cements  dissolve  less  easily  and  quickly  and  sand- 
stones with  siliceous  cements,  especially  the  quartzites,  are  very  resistant 
to  weathering,  especially  solution.  In  fact  the  weathering  of  quartz- 
ites and  sandstones  with  siliceous  cements  is  mainly  by  disintegration; 
instead  of  breaking  into  the  original  grains,  the  rock  tends  to  break 
into  flakes  and  angular  fragments  and  to  yield  a  shallow,  gravelly  soil. 
Sandstones  for  the  most  part  readily  yield  to  disintegration.  Many 
sandstones  are  stratified  and  the  planes  of  stratification  are  planes  of 
weakness  and,  moreover,  sandstones,  as  a  rule,  are  porous  and  absorb 
considerable  quantities  of  water  which,  in  freezing  rifts  and  flakes  the 
rock. 

QUARTZITE  SOILS 

Quartzites  are  not  widespread  and  so  far  as  area  is  concerned  they 
are  not  important  as  soil  makers.  Owing  to  their  hardness  and  tenacity 
these  rocks  are  extremely  resistant  to  erosion  and  almost  always  underlie 
rough  and  relatively  high  country.  Again,  their  slow  weathering  makes 
for  a  slow  formation  of  soil  so  that  quartzite  soils  are  usually  shallow, 
siliceous  and  unproductive.  Stony  loams  are  perhaps  the  predominant 
type. 


92 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


Fig.  75  shows  a  section  of  the  closely  folded  metamorphic  rocks  of  the  Piedmont 
Plateau  in  southeastern  Pennsylvania.  The  Chester  series  is  for  the  most  part 
derived  from  schists;  the  soils  are  known  locally  as  "gray  lands."  The  Chester 
loam  in  this  area  is  derived  from  granite-like  gneiss.  Both  soil  and  subsoil  are  silty 


FIG.  75. — Diagram  to  show  the  occurrence  of  rocks  and  their  derived  soils  on  the 
Piedmont  in  Pennsylvania.  The  rocks  are  closely  folded  and  include  gneiss 
(G),  quartzite  (Q)  and  limestone  (L).  Length  of  section  about  seven  miles. 
(Data  from  U.  S.  Bureau  of  Soils  and  U.  S.  Geological  Survey.) 

and  typically  they  contain  considerable  amounts  of  stony  fragments.  The  quartzitic 
sandstone  weathers  relatively  slowly  and  yields  a  stony  loam  of  low  fertility  and  a 
hilly  topography.  The  limestone  yields  loams  and  silt  loams  of  the  Hagerstown  series. 


SHALE   AND   SLATE  SOILS 


SHALE  SOILS 

Shales,  like  sandstones,  are  composed  of  materials  which  are  the 
results  of  long-continued  weathering  and  are,  therefore,  but  little 
affected  by  decomposition.  The  clays,  their  principal  constituent, 
have  been  derived  from  the  more  or  less  thorough  weathering  of  other 
rocks  and  are  comparatively  stable.  The  main  evidence  of  weathering 
in  shales  is  the  frequent  oxidation  of  their  iron  compounds  which  results 
in  reddish  and  yellowish  colors.  Shales  practically  always  contain 
considerable  sand  and  they  grade  into  the  sandy  shales  and  into  the 
shaly  sandstones.  On  the  other  hand,  the  shales  may  become  more 
calcareous  and  grade  into  shaly  limestone.  Many  of  the  black  calcareous 
soils  of  the  Gulf  Coastal  Plain  are  derived  from  these  calcareous  shales. 
Shales  are  often  interstratified  with  layers  of  sandstone  or  sandy  shales 
in  which  the  sand  grains  are  small  and  these  are  not  infrequently  inter- 
spersed with  thin  beds  of  limestone,  and  the  weathering  of  such  strata 
frequently  yields  heavy  loams  or  silt  loams.  Shales  which  are  finely 
stratified  tend  to  disintegrate  into  small  flat  plates  which  weather 
slowly  and  remain  in  the  soil,  yielding  shale  loams.  Such  fragments 
also  occur  on  slopes  where  the  finer  materials  have  been  washed  away. 


SHALE  AND  SLATE  SOILS 


93 


Mineralogical  analyses  of  two  soil  series  derived  from  sandstones 

and  shales  are  as  follows:  1 


MINERALS  OTHER 

THAN    QUARTZ    IN 

ABUNDANT  AND  CHARACTERISTIC  MINERALS  IN 

Sand, 
per  cent. 

Silt, 
per  cent. 

Sand. 

Silt. 

Penn  series  .  .  . 

5 

20 

Orthoclase  much  al- 

Decomposed 

feldspar. 

tered,  hematite. 

Dekalb  series. 

2-3 

8 

Orthoclase  very  much 

Tourmaline. 

altered. 

The  point  of  particular  interest  in  the  table  is  the  greater  amount  of 
other  minerals  than  quartz  in  the  Penn  series,  and  this  has  a  close  rela- 
tion to  the  origin  of  the  parent  rocks.  The  Penn  series  is  derived  from 
Triassic  sandstones  and  shales,  and  these  rocks  were  largely  derived 
from  the  wear  of  granites  and  from  other  igneous  rocks  which  were  not 
far  distant.  Therefore  the  Penn  series  naturally  shows  a  high  per- 
centage of  undecomposed  minerals,  especially  the  feldspars.  The 
Dekalb  soils,  derived  from  sandstones  and  shales,  are  important  soils 
east  of  the  Mississippi.  They  show  a  low  per  cent  of  minerals  other 
than  quartz. 


SLATE  SOILS 

Slates  weather  in  much  the  same  way  and  to  much  the  same  final 
products  as  shales.  This  is  expectable  since  slates  are  for  the  most  part 
metamorphosed  from  shales.  Micas  are  usually  developed  during  the 
metamorphism  from  shales  to  slates  but  generally  the  grains  are  so 
small  that  they  have  little  effect  on  the  soils  except  to  make  them  some- 
what more  open  textured.  Quartz  is  often  formed,  as  in  gneisses  and 
schists,  in  long  bands  or  "  stringers,"  and  these  often  remain  in  the  soils 
in  considerable  quantities.  The  fine  cleavages  of  slate  render  it  easy 
for  the  action  of  disintegrating  processes  so  that  the  slate  breaks  up  into 
thin,  flat  plates,  but  these  plates,  because  of  their  firmness  and  hardness, 
do  not  decompose  readily;  they  occur  in  practically  all  slate  soils  and, 
when  numerous,  they  give  rise  to  the  characteristic  slate  loams.  When 
completely  decomposed,  slates  yield  fine-textured  soils,  and  most  of  the 
slate  soils  that  have  been  described  are  silt  loams. 
1  McCaughey  and  Fry,  loc.  cit. 


94 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


Slate  outcrops  are  usually  narrow  and,  therefore,  are  not  important 
as  soil  makers  and,  furthermore,  these  outcrops  usually  occur  in  moun- 

r tainous  regions  of  highly  folded 

rocks  where  soils  are  of  little 
agricultural  importance.  There 
is  a  considerable  area  of  slate 
soils  in  the  North  Carolina 
slate  belt  and  some  of  these 


FIG.  76. — Residual  soils  from  slate,  diorite 
and  granite,  N.  C.  (Data  from  U.  S. 
Bureau  of  Soils.) 


soils  have  considerable  agri- 
cultural value.  The  soils  of  a 
portion  of  this  belt  are  shown 
in  Fig.  76  where  the  Allamance  series  is  derived  from  dense,  fine- 
grained, bluish  slates  and  which  yield  silt  loams. 

Comparison  of  Sedimentary  Rocks. — The  combined  compositions 
of  limestone,  shales  and  sandstones  are  shown  in  Fig.  77.     The  pre- 


LIMESTONES 


SHALES 


SANDSTONES 


FIG.  77. — Generalized  diagram  to  show  the  composition  of  limestones,  shales  and 
sandstones.     (Data  after  Clarke,  U.  S.  Geological  Survey.) 

ponderating  minerals  of  limestones  are  calcite  and  dolomite,  that  of 

sandstone  is  silica  with  a  small  amount  of  alumina  which  is  contained 

mostly  in  the  clay  and  a  still 

smaller    amount     of     calcite 

which  is  mainly  a  cement.     In 

shales,  the  alumina  and  silica 

are  to  a  great  extent  combined 

in  a  silicate,  kaolin,  which  is 

the  essential   mineral   of   the 

clays. 

Some  soil  belts  from  sediment- 
ary   rocks    are    found    in    eastern 
Kansas,  Fig.  78.     The  rocks,  dip- 
ping in  a  westerly  direction,  expose  their  outcrops,  which  yield  roughly  parallel  belts 
of  soils.     The  shale  yields  a  silt  loam  underlain  by  a  clay  subsoil.     Next  to  the 


FIG.  78. — Residual  soils  from  sedimentary 
rocks,  Kansas.  (Data  after  U.  S.  Bureau 
of  Soils.) 


SHALE  AND  SLATE  SOILS  95 

westward  is  a  formation  including  thin  beds  of  shales  and  fine-grained  sandstones 
which  yield  loams  and  sandy  loams.  Overlying  this  is  a  thick  bed  of  limestone 
marked  by  a  belt  of  deep  red-clay  loams.  The  uppermost  formation  is  a  friable 
sandstone  which  to  a  casual  observer  is  almost  pure,  but  it  contains  so  much 
clay  and  silt  that  its  soils  are  mainly  sandy  loams  instead  of  sands. 

REFERENCES— RESIDUAL   SOILS  FROM   SEDIMENTARY  ROCKS 

C.  F.  MARBUT,  Reconnoissance  Soil  Survey  of  the  Ozark  Region  of  Arkansas  and 
Missouri,  U.  S.  Bureau  of  Soils,  1911. 

U.  S.  Bureau  of  Soils,  Reconnoissance  Soil  Survey  of  Western  South  Dakota,  1909; 
South-central  Texas,  1913;  Southwestern  Texas,  1911;  South-central  Penn- 
sylvania, 1910;  Southwestern  Pennsylvania,  1909;  Southeastern  Pennsylvania, 
1912. 

GRANITE  AND  GNEISS  SOILS 

It  appears  from  geological  investigations  that  rocks  of  granitic  com- 
position have  the  greatest  surface  exposure  of  any  igneous  rocks  and  they 
are,  therefore,  very  important  as  soil  makers.  Granites  and  gneisses 
can  be  considered  together  because  their  composition  is  much  the  same. 
They  belong  to  the  acid  group  of  igneous  rocks,  that  is,  rocks  with 
high  percentages  of  quartz.  Both  rocks  have  in  considerable  abundance 
quartz  and  feldspars,  especially  orthoclase  (the  potash  feldspar)  and, 
to  a  less  extent,  micas,  hornblende  and  small  percentages  of  apatite, 
the  phosphorus-yielding  mineral.  The  gneisses,  however,  have  a 
much  larger  percentage  of  accessory  minerals  but  these  usually  compose 
only  a  small  percentage  of  the  entire  rock.  The  chemical  .weathering  or 
decomposition,  therefore,  of  both  granites  and  gneisses  can  well  be  con- 
sidered together. 

Weathering  of  Granites  and  Gneisses. — The  first  minerals  to  show 
weathering  are  usually  the  feldspars.  The  two  cleavages  of  these  min- 
erals are  double  lines  of  weakness,  and  even  when  the  minerals  are  com- 
paratively fresh  there  are  often  faint  whitish  lines  along  the  cleavages 
indicating  initial  decomposition.  That  the  feldspars  weather  rather 
readily  is  shown  in  many  fresh,  sound  granites  where  the  smooth  faces 
of  some  of  the  feldspars  appear  "  chalky."  This  appearance  indicates 
that  the  feldspars  are  beginning  to  decompose,  a  process  which  ends 
with  the  solution  of  the  alkaline  compounds  and  the  accumulation  of 
clay  and  sand  as  shown  in  the  equation  on  page  73.  Since  ortho- 
clase is  a  prominent  mineral  in  granites,  it  is  apparent  that  granitic 
soils  in  general  are  fairly  well  supplied  with  potash.  Hornblende  and 
mica  are  usually  found  in  varying  quantities  in  all  granites.  Horn- 


96 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


blende,  owing  to  its  easy  cleavages,  disintegrates  rather  quickly  and 
decomposes,  setting  free  iron  compounds,  clay,  sand  and  carbonates 
of  lime,  magnesia,  soda  and  potash.  Mica,  while  it  disintegrates  easily, 
is  slow  to  decompose  and  often  remains  in  considerable  quantities  in 
the  soils.  Muscovite,  the  potash  mica,  upon  final  decomposition,  yields 
a  small  quantity  of  potash. 

Both  gneisses  and  granites  readily  disintegrate  into  coarse  particles, 
the  process  being  aided  by  the  variety  of  minerals  and  their  various 


FIG.  79. — The   change  from  fresh  to  weathered  granite  and  to  soil,  Md.     (Md. 

Geological  Survey.) 


colors  which  tend  to  produce  differential  expansion  and  contraction. 
In  addition,  the  gneisses  have  a  markedly  schistose  structure  which 
causes  the  rock  to  split  rather  readily  along  the  planes  of  weakness. 
The  quartz  in  gneisses  is  very  likely  to  occur  in  bands  or  "  stringers  " 
instead  of  in  grains  as  in  granite.  Soils  from  gneisses  are,  therefore, 
likely  to  contain  these  undecomposed  masses  of  quartz  as  gravel,  locally 
in  such  quantities  as  to  render  the  soil  sandy  or  gravelly.  From  this 
feature  it  is  often  easy  to  determine  the  gneissic  origin  of  a  soil  even 
when  the  bed  rock  cannot  be  seen. 


SHALE  AND  SLATE  SOILS 


97 


Chemical  and  Mineralogical  Changes. — The  general  facts  as  to  the 
weathering  of  granites  into  soils  are  illustrated  in  Fig.  80,  in  which  the 
compositions  of  fresh  rock  and  its  residuum  are  compared.  The  rock,  as 
described  by  Watson,  is  a  fine-grained,  blue-gray  granite  showing  quartz, 
feldspar,  biotite  and  muscovite.  The  weathered  zone  is  about  20  feet 
in  depth,  grading  from  fresh  granite  upward  into  reddish,  discolored, 
crumbly  granite  and,  finally,  into  a  deep  red  clay,  somewhat  gritty, 
owing  to  the  particles  of  quartz  and  undecomposed  feldspar.  The 
feldspars  in  the  weathered  zone  take  on  whitish  and  chalky  appearances 
due  to  the  partial  change  to  kaolin  and  the  micas  become  brittle  and 
bleached.  In  the  upper  zone  these  minerals  are  mostly  decomposed 
into  residual  clay. 

From  the  diagram  we  note  that  there  is  less  silica  and  more  alumina 
in  the  residual  clay  than  in  the  fresh  rock.  A  portion  of  the  silica  which 


V1 

SILICA           69-88£ 
ALUMINA       16'42/< 
IRON  OXIDE    1l96$ 
LIME                  1-78£ 
POTASH            *-46?, 
SODA                 6-63^ 
WATER               °-36^ 

f 

)                            2 

RESH  GRANITE 
5%                       '^  \ 

n                      75% 

RESIDL 
0                           2 

rAL  CLAY 

#*       '.^    :                  5 

0% 

54.  Iff, 
25.90% 
4.69$ 
0.05$ 
2.16% 
2.87$ 
10.1*$ 

i,^^,.^ 

—  *•-  '•~^--i-/-l 

— 

— 

2 

3 

m  

! 

S3  

| 

FIG.  80. — Diagram  to  illustrate  the  chemical  composition  of  a  granite  and  its  resid- 
ual clay.     (Data  after  Watson.) 


existed  in  the  rock  as  quartz  has,  doubtless,  gone  into  solution  and  been 
lost.  The  higher  percentage  of  alumina  is  due  in  part  to  the  fact  that 
there  is  a  higher  percentage  of  clay  in  the  residuum.  In  this  case  the 
iron  oxides  are  higher  in  the  clay  than  in  the  fresh  rock  for  the  iron- 
bearing  minerals,  mostly  biotite,  in  the  granite  have  decomposed  in 
part  to  iron  oxides  which  are  rather  insoluble  and  surround  the  sand  and 
clay  grains  mostly  as  thin  films.  The  lime,  soda  and  potash  are  decid- 
edly less  in  the  clay  owing  to  the  solubility  of  their  weathered  com- 
pounds, mostly  carbonates.  Furthermore,  some  of  the  lime,  soda  and 
potash  yet  remain  in  the  form  of  the  original  feldspars  which  are  as  yet 
undecomposed  and  remain  in  the  clay,  where  they  slowly  decompose  and 
yield  valuable  plant  foods.  This  slow  decomposition  explains  the 
"  staying  qualities  "  of  most  granitic  soils.  The  marked  increase  in 
water  in  the  clay  is  due  to  the  fact  that  clay  is  a  hydrated  mineral,  for  in 


98 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


its   formation   water   has   united    with   alumina   and    silica   to   form 
the  clay. 

The  mineralogical  composition  of  the  Cecil  soils  of  North  Carolina 
which  are  derived  from  granitic  rocks  is  given  below.1 


PERCENTAGE  OF  MINERALS  NOT 

QUARTZ    IN 

ABUNDANT  MINERALS,  NOT  QUARTZ,  IN 

Sand, 
per  cent. 

-       Silt, 
per  cent. 

Sand. 

Silt. 

30 

34 

Orthoclase,  muscovite,  bi- 

Muscovite,  biotite,   ortho- 

otite,    epidote,    micro- 

clase,  epidote, 

microline. 

line. 

This  soil  is  found  mostly  on  the  rolling  surface  of  the  Piedmont. 
It  is  thoroughly  weathered  and  of  a  reddish  color,  especially  in  the  sub- 
soils. In  spite  of  the  extensive  de- 
composition there  are  considerable 
quantities  of  feldspars  and  mica  in  the 
sand  and  silt.  The  mica  is  especially 
abundant  in  flakes  of  microscopic  size 
in  the  silts,  Fig.  81.  These  fine  par- 
ticles of  feldspar  and  mica  are  of  great 
value  in  supplying  potash  and  lime  to 
the  soils. 

Notable  Regions. — While  granites 
have  wide  surface  exposures  in  North 
America,  the  areas  in  the  Rocky 
Mountain  region  are,  for  the  most 
part,  hilly  and  not  of  much  agri- 
cultural value.  However,  the  rolling 

Piedmont  in  the  eastern  part  of  the  United  States  includes  large 
areas  of  productive  soils  derived  from  granites  and  gneisses.  Two  great 
soil  series  here  are  largely  derived  from  granites,  the  Cecil  series  in  the 
southern  Piedmont  and  the  Chester  in  the  northern  Piedmont.  These 
soils  are  characterized  by  red,  heavy  subsoils  with  angular  quartz  grains 
scattered  through  soil  and  subsoil  and  mica  is  common  in  the  sub- 
soils. The  Durham  series  is  siliceous  and  is  derived  from  acid  granites. 

1  J.  K.  Plummer,  Journal  of  Agricultural  Research,  Vol.  5,  Part  1,  1915-16, 
pages  569-581. 


FIG.  81. — Microphotograph  of  the 
soil  from  igneous  rocks  containing 
biotite  mica.  Most  of  the  dark 
minerals  are  biotite.  N.  C.  (Plum- 
mer, N.  C.  Experiment  Station.) 


SHALE  AND  SLATE  SOILS 


SOILS  FROM  BASIC  ROCKS — DIORITE  AND  BASALT 

Introductory. — It  will  be  remembered  that  these  rocks  are  more  basic 
than  the  granites,  that  is  they  contain  more  iron,  lime,  soda  and  magnesia 
and  less  quartz.  In  these  rocks  there  are  relatively  large  amounts  of 
hornblende,  biotite,  olivine  and  those  feldspars  which  contain  con- 
siderable lime  and  soda.  The  rocks  are  typically  dark  gray  to  black  in 
color  and  are  usually  hard  and  heavy.  Locally  they  are  often  known 
as  "  black  rocks  "  or  "  greenstones."  Diorite  is  a  rather  common  rock 
in  many  localities,  especially  in  the  southern  Piedmont  and,  it  will  be 
remembered,  there  is  a  very  large  area  of  basalt  in  the  Columbia  River 
region,  Fig.  28. 

Weathering  in  General. — In  general,  rocks  high  in  lime,  iron  and  soda 
decompose  more  readily  than  the  granites,  which  are  more  siliceous,  but 
this  is,  however,  subject  to  many  variations  since  rock  structure  and 
texture  are  potent  factors  in  weathering.  Then  nearly  all  the  promi- 
nent minerals  in  these  rocks  possess  easy  cleavages  which  facilitate 
disintegration  and  thereby  allow  free  play  to  the  agents  of  decomposi- 
tion. Again,  the  dark  coltfrs  of  these  rocks  is  a  persistent  factor  favoring 
disintegration.  It  will  readily  be  apparent  that,  since  the  feldspars 
ultimately  weather  to  clay  and  the  hornblende  and  allied  minerals 
weather  to  clay  and  iron  compounds,  the  soils  derived  from  these  rocks 
are  clays  strongly  colored  with  the  oxidized  compounds  of  iron — heavy 
reddish  soils  with  but  little  quartz. 

Diorites  and  Their  Soils. — Diorite  soils  are  important  in  the  southern 
Piedmont.  They  have  heavy  subsoils  of  sticky  yellowish  clay  or  clay 
loam  with  but  little  quartz.  The  subsoils  pass  below  into  weathered 
diorite  or  similar  rocks. 

The  chemical  composition  of  these  soils  is  illustrated  in  Fig.  82, 
which  shows  interesting  comparisons  between  fresh  and  decomposed 
diabase,  a  rock  somewhat  similar  to  diorite  but  more  basic.  The  fresh 
rock  ha&  a  coarse,  granular  texture  with  a  dark-gray  color  and  greasy 
luster.  Feldspar,  augite,  and  some  magnetite  and  olivine  can  be  recog- 
nized by  the  unaided  eye  and  these  minerals  account  for  the  rather 
high  percentages  of  iron,  lime,  magnesia  and  soda  in  the  fresh  rock. 
The  residuum  is  an  orange-colored  clay  with  most  of  the  lime  and 
magnesia  leached  out  as  soluble  carbonates.  The  alumina  and  silica 
have  not  changed  greatly  since  they  are  present  in  the  residual  clay. 
The  very  high  percentage  of  iron  in  this  case  is  due  to  the  fact  that  much 


100 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


of  the  magnetite  (FesCU)  and  some  augite,  both  iron-bearing  minerals, 
remain  undecomposed  as  small  grains.  The  feldspar  has  nearly  all 
been  changed  to  clay. 


FRESH    DIABASE 


DECOMPOSED   DIABASE 
2.5/0 


SILICA  45.73^ 

ALUMINA  13.48# 
IRON  OXIDES  11.60# 
LIME  9.92# 

MAGNESIA  15.40^ 
SODA  3.24^ 

WATER  0.  94 % 


FIG.  82. — Diagram  to  illustrate  the  chemical  composition  of  fresh  and  weathered 
diabase.     (Data  after  Merrill.) 

The  following  mineralogical  analysis  of  soils  from  basic  rocks  (Ire- 
dell  series)  illustrates  some  of  the  features  of  these  soils : 1 


MINERALS  OTHER  THAN 
QUARTZ  IN 


ABUNDANT  AND  CHARACTERISTIC  MINERALS  IN 


Sand, 
per  cent. 

Silt, 
per  cent. 

Sand. 

Silt. 

30 

40 

Augite, 
dote. 

hornblende,  epi- 

Biotite, 
blende. 

epidote,   horn- 

This  soil  is  derived  from  basic  igneous  and  metamorphic  rocks, 
a  derivation  that  is  evident  from  the  high  percentages  of  the  basic  iron- 
bearing  minerals  that  persist  in  the  soil.  The  feldspars  have  practically 
all  been  decomposed  both  in  the  sands  and  in  the  silts,  owing  in  part 
to  their  comparatively  easy  solubility. 

Basalts  and  Their  Soils. — Basalts  are  important  soil-forming  rocks 
in  some  localities.  The  term  is  somewhat  general;  ordinary  basalts 
are  porphyritic  and  olivine  ((MgFe)2SiO4)  is  a  common  phenocryst, 
but  often  basalts  are  dense  and  only  minutely  porphyritic.  Many 
volcanic  flows,  both  recent  and  ancient,  are  basaltic,  and  most  active 
volcanoes  eject  basaltic  lavas.  Basaltic  lavas  are  usually  porous,  a 
feature  which  helps  in  rapid  weathering,  and  this,  together  with  the  basic 
composition  which  is  favorable  for  weathering,  causes  such  lavas  quickly 
to  yield  productive  soils.  In  fact,  vines  are  often  set  out  on  recent  lavas 
1  McCaughey  and  Fry,  loc.  cit. 


SHALE  AND  SLATTE  'SOILS  101 

from  Vesuvius  only  a  few  years  after  eruptions.  The  basalts  of  the 
Columbia  River  flow,  Fig.  28,  are  lava  flows  rather  recent  from  a  geo- 
logical point  of  view.  They  are  weathered  in  places  to  a  depth  of 
60  or  more  feet  to  a  reddish  mantle  rock  which  often  yields  productive 
soils.  Basalts  are  often  found  in  dikes,  sills  and  other  forms  of  volcanic 
origin. 

OBSIDIAN  SOILS 

These  glassy  rocks  are  not  important  soil  makers,  both  because  they 
are  not  of  wide  extent  and  because  the  soils  are  generally  infertile.  The 
obsidians  occur  in  considerable  areas  in  recent  lava  flows,  notably 
in  the  region  of  the  Columbia  River  lavas,  Fig.  28.  Here  for  the  most 
part  the  obsidian  areas  are  but  little  weathered,  the  soils  are  thin  and 
the  areas  are  almost  destitute  of  vegetation.  The  obsidians  are  notably 
resistant  to  weathering,  especially  decomposition,  for  it  is  mostly  by  dis- 
integration that  they  are  changed  into  soils  and  the  process  is  very  slow. 
Sharp,  angular  sand  is  usually  formed  and  this  may  change  into  soil  by 
long-continued  weathering. 

SCHIST  SOILS 

Schists  vary  greatly  in  composition  but  in  general  they  are  basic 
rather  than  acid.  They  have  a  great  variety  of  minerals  which  have 


FIG.  83. — Soils  mostly  from  mica  schists,  Pa.     (U.  S.  Bureau  of  Soils.) 

been  developed  by  metamorphism,  among  them  being  micas,  horn- 
blende, garnet  and  feldspars.  Talc  is  a  frequent  mineral  and  gives  a 
characteristically  greasy  feel  to  many  soils  from  schists.  Mica  and 


102  RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 

hornblende  schists  are  by  far  the  most  common  and  are  the  only  ones  of 
much  importance  as  soil  makers.  Like  gneisses,  schists  often  have 
long  inclusions  or  "  stringers  "  of  quartz  and  feldspars  which  remain  un- 
decomposed  in  the  soil.  Schists  are  typically  rather  infirm  and  easily 
broken  up  and  one  can  often  pick  weathered  mica  schists  to  pieces  with 
the  fingers.  Naturally  such  rocks  yield  readily  to  disintegration  and 
this  is  especially  true  of  mica  schists,  which  are  often  so  thoroughly 
broken  up  that  the  mica  flakes  are  carried  by  the  streams  and  blown 
about  by  the  winds  and  are  known  locally  as  "  sand."  Schists  are 
usually  associated  with  gneisses  and  often  with  slates.  The  mica 
schists,  although  they  disintegrate  very  readily,  do  not  decompose 
rapidly,  for  the  mica  is  resistant  to  chemical  weathering.  When  subject 
to  complete  weathering,  schists  give  rise  to  heavy  soils  usually  with  a 
high  content  of  reddish  clay.  The  talc  and  very  small  mica  grains  in 
some  schists  often  give  a  "  greasy  "  feel  to  these  soils. 

REFERENCES 

H.  H.  BENNETT,  Soils  of  the  Piedmont  Plateau  Province  in  Soils  of  the  United  States, 
Bull.  96,  U.  S.  Bureau  of  Soils,  1913;  General,  pages  9-21,  Soil  Series,  pages 
22-48  (mostly  from  igneous  and  metamorphic  rocks). 

General  Weathering.  CHAMBERLIN  AND  SALISBURY,  Geology,  Vol.  1,  Holt,  1904, 
Chapter  2. 

JAMES  GEIKIE,  Earth  Sculpture,  Putnam,  1898,  Chapter  2. 

A.  W.  GRABEAU,  The  Principles  of  Stratigraphy,  Seiler,  1913;  The  Atmosphere, 
pages  24-31;  Geological  Work  of  Heat  and  Cold,  pages  31-34;  Chemical  Work 
of  the  Atmosphere,  pages  34-40. 

E.  W.  HILGARD,  Soils,  Macmillan,  1911,  Chapters  3-4. 

LYON,  FIPPEN  AND  BUCKMAN,  Soils,  Macmillan,  1916,  Chapter  5. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  Chapter  on 
Weathering. 

R.  D.  SALISBURY,  Physiography,  Holt,  1907,  Chapter  2. 

TARR  AND  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapter  2. 

Residual  Soils.  S.  C.  JONES,  Kentucky  Soil  Survey,  Kentucky  Geological  Survey, 
1913. 

W.  N.  LOGAN,  The  Soils  of  Mississippi,  Technical  Bulletin  4,  Miss.  Experiment  Sta- 
tion, 1913. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Chapters  on  Residuary 
Deposits. 

E.  H.  SELLERDS,  Soils  of  Florida,  4th  Ann,  Rept,,  Florida  Geological  Survey,  1912. 


INHERITED  SOILS  103 

INHERITED  SOILS 

It  will  be  apparent  that  a  residual  soil  is  not  derived  from  the  under- 
lying rock  as  is  sometimes  stated,  but  rather  from  rock  which  has 
disappeared  by  changing  into  soil  and  that  this  former  rock  at  one  time 
overlay  the  present  bed  rock.  In  other  words,  most  residual  soils  have 
been  derived  from  weathering  of  rocks  which  overlay  the  present  soil 
and,  in  a  sense,  the  soils  are  inherited  from  rocks  which  have  disappeared. 
An  exception  to  this  inheritance  occurs  when  rocks  are  newly  exposed 
such,  for  example,  as  the  soils  from  a  recent  volcanic  flow  when  the  soil 
is  due  only  to  the  weathering  of  the  fresh  rock  surface.  However, 
the  statement  holds  true  in  most  cases  that  residual  soils  are  derived 
from  more  or  less  thick  rock  strata  which  have  disappeared  through 
weathering.  The  term  inherited  soils  is  useful  only  when  the  soils 
have  peculiarities  or  characteristics  which  are  due  to  some  formerly 
overlying  stratum. 

In  humid  climates  at  least,  the  residual  soils  are  the  residue  of  rocks 
the  soluble  portions  of  which  have  been  carried  away.  The  insoluble 
residue  of  one  rock  stratum  sinks  down  to  mingle  with  the  residue  of 
the  underlying  strata  as  the  weathering  goes  on.  It  is  true  that  some 
insoluble  materials,  especially  the  finer  particles,  are  washed  away,  but 
practically  all  residual  soils  due  to  the  weathering  of  considerable  thick- 
nesses «of  rocks  contain  materials  from  rocks  that  were  formerly  over- 
lying. 

A  fine  example  of  inheritance  on  a  large  scale  is  found  in  the  Blue 
Grass  region  of  Kentucky,  Fig.  84.  The  present  soils  are,  as  a  whole, 
distinctively  limestone  soils  and 
are  underlain  by  limestone.  The 
limestone  yielding  the  recent  soils 
is  somewhat  pure  and  yields  silts  FIG.  84. — Diagram  of  the  Blue  Grass 
and  clays.  However,  the  soils  Region  of  Kentucky  to  illustrate  soil 
contain  fragments  of  chert  which,  inheritance.  (After  Shaler.) 
being  relatively  insoluble,  have 

descended  from  a  higher  lying  limestone  stratum,  the  horizon  of 
which  is  fully  a  thousand  feet  above  the  present  soil.  Chert  fossils 
are  found  in  the  present  soil,  which  are  characteristic  of  this  high- 
lying  limestone. 

It  is  perhaps  worth  while  to  imagine  the  successive  residual  soils 
which  have  been  formed  in  the  region  shown  in  Fig.  84.  The  soils  from 
the  upper  limestone  (LS)  were  doubtless  fertile  and  somewhat  heavy 


104 


RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 


clays  and  silt  loams.  As  the  limestone  wore  away  the  soils  from  the 
sandstone  (SS)  succeeded,  at  first  more  or  less  commingled  with  the 
clays  and  silts  from  the  limestone,  but  gradually  assuming  a  sandy 
texture  and  doubtless  supporting  a  vegetation  characteristic  of  sandy 
soils.  Later  the  soil  zone  descended  to  the  shale  (S)  when  a  heavy,  stiff 
soil  appeared.  At  first  the  soils  from  the  upper  layers  of  shales  included 
some  sand  from  the  preceding  sandstone  soils  and  was  perhaps  a  loam  or 
even  a  sandy  loam,  but  later  the  texture  became  heavier  and  the  soil 
assumed  the  characteristics  of  a  heavy  shale  soil.  Lastly,  of  course,  the 
soil  zone  has  reached  the  limestone  and  the  present  soils  have  inherited 
more  or  less  characteristic  materials  from  the  three  formerly  overlying 
formations. 

Another  example  of  soil  inheritance  is  shown  in  Fig.  85,  an  area  in 
Oklahoma  where  the  present  soils  are  markedly  influenced  by  formerly 


FIG.  85. — The  diagram  to  the  left  shows  the  Tishomingo  formation  (TF]  overlying 
granite  (G}.  In  the  right-hand  diagram,  the  Tishomingo  formation  has  been 
mostly  eroded  with  the  result  that  inherited  soils  cover  the  upper  part  of  the 
diagram,  Okla.  (Data  from  U.  S.  Geological  Survey  and  U.  S.  Bureau  of 
Soils.) 

overlying  strata.  The  sea  advanced  over  the  surface  of  the  granite  (G) 
and  deposited  the  Trinity  Formation  (TF),  mostly  sandstone  but  con- 
taining layers  of  gravel  in  the  lower  portions.  After  this  there  was 
an  uplift  which  increased  the  activity  of  the  streams  so  that  erosion  has 
stripped  away  much  of  the  Trinity  Formation,  again  exposing  the  old, 
formerly  buried  granite  surface  in  many  places.  As  a  result,  some 
soils  are  derived  from  the  old  exposed  granite  and  some  from  the  sandy 
and  gravelly  Trinity  Formation.  Still  other  soils  are  inherited  and 
include  materials  from  both  formations. 

Widely  distributed  but  disconnected  areas  of  inherited  soils  occur 
near  the  junction  cf  the  Piedmont  Plateau  and  the  Coastal  Plain, 
Fig.  86.  The  Coastal  Plain  is  composed  largely  of  sand  and  clay,  for 
the  most  part  unconsolidated,  and  the  strata  dip  gently  away  from  the 
Piedmont  Plateau.  On  the  other  hand  the  Piedmont  Plateau  is  mostly 


INHERITED  SOILS 


105 


underlain  by  hard,  consolidated  rocks.  Formerly  the  Coastal  Plain 
extended  further  back  a  considerable  distance  over  the  Piedmont 
Plateau  but  erosion  has  removed  most  of  the  finer  Coastal  Plain  materials 
so  that  only  some  gravels  and  sands  have  remained  on  the  Piedmont 
Plateau  to  modify  its  soils.  A  case  in  point  is  illustrated  in  Fig.  86. 


PIEDMONT  PLATEAU 


COSTAL  PLAIN 


FIG.  86. — Diagram  of  the  junction  of  the  Coastal  Plain  and  Piedmont  Plateau.  The 
dotted  line  shows  the  junction.  The  dotted  areas  indicate  formerly  overlying 
Coastal  Plain  materials  which  have  not  been  entirely  eroded.  (Adapted  from 
U.  S.  Geological  Survey.) 

Small  patches  of  sand  and  gravelly  material  yet  remain  on  the  Piedmont 
Plateau  as  inheritances  from  the  formerly  overlying  Coastal  Plain  while 
still  larger  areas  contain  coarse  materials  from  formerly  overlying 
Coastal  Plain  Formations.  Finally  there  are  fine  examp  es  of  soil 
inheritance  in  soil  belts  surrounding  the  eastern  Ozarks,  an  example 
of  which  is  shown  in  Fig.  87.  The  rocks  here  are  gently  dipping 


FIG.  87.— Diagram  to  illustrate  inherited  soils.  The  successive  formations  have 
contributed  to  the  soils  to  the  westward  (left).  (LS}  limestone;  (SS)  sand- 
stone; (S)  shales;  (CLS)  cherty  limestone.  Section  about  25  miles  long. 
(Data  from  U.  S.  Bureau  of  Soils.) 


and  such,  soils  often  show  marked  inheritance.  The  soils  in  this 
area  are  practically  all  silt  loams,  but  most  of  the  soil  belts  show 
marked  influences  due  to  formerly  overlying  rocks.  Each  formation 
formerly  extended  westward  at  higher  elevations  than  at  present  and, 
as  a  formation  was  worn  away,  its  coarser  and  more  insoluble 
materials  sank  and  constitute  a  part  of  the  soils  now  overlying 
rocks  to  the  westward.  Such  a  structure  causes  the  different  soil 
types  to  grade  into  each  other  and  to  resemble  the  soil  belt  on  the 


106  RESIDUAL  SOILS  FROM  VARIOUS  ROCKS 

next  adjoining  formation  to  the  eastward.  For  example,  the  soil  from 
the  sandstone,  (SS,  2)  is  much  like  the  soil  from  the  next  limestone  belt 
to  the  east  (LS,  3)  but  the  subsoil  is  what  one  would  expect  from  the 
sandstone  alone.  Again,  the  insoluble  cherts  from  the  cherty  limestone 
(CLS,  5)  are  scattered  over  types  to  the  westward  types  which  were 
once  overlain  by  this  formation.  ^ 

REFERENCE 

N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Report,  Part  1,  1890-91,  U.  S. 
Geological  Survey,  Soil  Inheritance,  pages  300-306. 


CHAPTER  V 
WIND  WORK  AND  EOLIAN  SOILS 

Introductory. — One  living  in  a  humid  climate  is  likely  to  under- 
estimate the  importance  of  wind  work,  but  in  dry  regions,  where  there 
is  an  abundance  of  loose  materials  for  the  winds  to  move,  the  effects 
of  winds  are  much  in  evidence.  Dust  storms  are  not  uncommon  even 
in  humid  regions  and  a  strong  wind  may,  in  a  few  hours,  carry  away 
xg-g-  of  an  inch  of  soil.  At  this  rate  it  would  require  only  one  hundred 
winds  to  move  an  inch  of  loose  materials  and  only  1200  winds  to  move 
a  foot,  a  rate  that,  from  a  geological  point  of  view,  is  comparatively 
rapid.  From  the  viewpoint  of  soils,  wind  work  is  very  important  in 
all  climates,  for  the  effects  are  confined  to  the  surface  of  the  upper  mantle 
rock  which,  of  course,  constitutes  the  soil.  Wind  work  is  practically 
universal  the  world  over  and,  furthermore,  as  has  been  noted  under 
volcanoes,  dust  may  be  carried  by  upper  air  currents  around  the  earth. 
The  Sirocco,  a  hot,  dry  southerly  wind  in  Italy,  brings  reddish  dust 
from  the  Sahara  in  northern  Africa,  a  distance  of  hundreds  of  miles, 
and  this  dust  mingling  with  rain  sometimes  causes  the  so-called  "  bloody 
rain  "  of  this  region.  Indeed,  "  it  would,  perhaps,  be  an  exaggeration 
to  say  that  every  square  mile  of  land  surface  contains  particles  of  dust 
brought  to  it  by  the  wind  from  every  other  square  mile,  but  such  a 
statement  would  probably  involve  much  less  exaggeration  than  might 
at  first  be  supposed  "  (Chamberlin  and  Salisbury).  Wind  work  is  an 
example  ,of  the  cumulative  effectiveness  of  apparently  insignificant 
agents  which  operate  persistently  and  for  a  long  time. 

Atmospheric  dust  is,  in  large  measure,  due  to  wind  work.  Its 
universal  presence  is  shown  by  a  shaft  of  sunlight  in  a  darkened  room. 
The  presence  of  air  dust  is  also  shown  by  melting  snow  which  leaves  a 
very  thin  layer  of  dust.  Indeed,  one  of  the  agricultural  benefits  of  snow 
is  the  fine  dust  which  is  left;  in  exceptional  cases,  A  to  Y^  of  an  inch  of 
dust  has  been  left  by  the  melting  of  a  heavy  snowfall. 

107 


108 


WIND  WORK  AND  EOLIAN  SOILS 


WIND  TRANSPORTATION 

Wind  transportation,  like  water  transportation,  is  largely  con- 
ditioned by  velocity,  but  since  air  is  only  about  ^3  as  heavy  as  water, 
the  striking  force  of  winds  is"  very  much  less  than  that  of  water.  Whirl- 
winds and  eddies  in  the  air  carry  dust  upwards  and  strong  horizontal 
winds  strike  dust  particles  and  project  them  on  a  journey  somewhat 
like  that  of  a  ball  from  a  bat.  Wind  load,  however,  is  almost  invari- 
ably very  fine,  seldom  but  a  fraction  of  a  millimeter  (about  •£$  inch)  in 


FIG.  88. — A  stratum  of  white  volcanic  dust  (pumicite)  9  feet  thick  lies  between  strata 
of  clay  about  the  middle  of  the  hill.  The  pumicite  is  volcanic  dust  and  is 
believed  to  have  been  transported  hundreds  of  miles  by  the  winds.  (E.  HL 
Barbour3  Neb,  Geological  Survey.) 

diameter.  The  importance  of  wind  as  compared  to  water  transporta- 
tion lies  in  the  broad  sweep  of  the  winds  which  pass  over  hills  and 
valleys  and  are  but  little  affected  by  slopes.  It  is  interesting  in  this 
connection  to  note  that  Udden  estimates  the  transporting  power  of  the 
winds  over  the  Mississippi  Basin  to  be  something  like  one  thousand 
times  as  great  as  the  transporting  power  of  the  Mississippi  itself.  The 
pumicite  deposits  of  Nebraska  and  adjoining  states  are  examples  of  the 
effectiveness  of  wind  transportation  over  long  distances,  Fig.  88. 
This  material  is  very  fine  volcanic  dust,  Fig.  25,  which  must  have 


DUNES 


109 


been  blown  hundreds  of  miles,  for  there  is  no  possible  source  nearer  than 
the  Rocky  Mountain  region  to  the  westward. 


Dunes 

are  hills  of  sand  that  are  moving  or  have  been  moved  by  the  wind. 
Given  a  small  obstacle  like  a  clump  of  bushes  or  a  rock,  the  winds 
are  checked  and  some  of  their  load  is  dropped  and  this  dropped  load 
in  turn  is  an  obstruction  which  leads  to  further  deposition  by  the 
wind.  Dunes  move  with  the  wind  because  the  winds  blow  the  sand  up 
the  windward  side  of  the  dune  and 
the  sand  then  drops  down  the  lee- 
ward side  as  shown  in  Fig.  89. 
Thus  dunes  will  advance  over  a 
region  and  often  when  they  move  FIG.  89.— Profile  of  a  dune.  The  arrows 
over  forests  they  leave  in  their  show  wind  directions, 

wake    the    deadened   trees   which 

are  appropriately  called  "  tree  graveyards. "  Dunes  are  often  a 
menace  to  fields  in  their,  path  and  to  roads  and  railroads  which 
they  sometimes  obstruct.  The  rate  of  dune  movement  varies  greatly. 
Most  dunes  move  but  a  few  inches  a  year,  but  they  have  been 
known  to  move  70  feet  or  more  annually. 

Several   factors   affect   dune   formation.     (1)    First,    of   course,   is 
an  abundant  supply  of  loose  materials,  usually  of  sand,  but  sometimes 

of   silt   which   the  wind   can 

move.  For  this  reason,  dunes 
of  various  dimensions  are 
often  found  on  sandy  beaches 
and  in  regions  where  the 
soils  are  very  sandy.  They 
are  especially  characteristic 
of  arid  regions  where  sand 
is  easily  moved.  (2)  Con- 
stancy of  wind  direction  is  an 
important  factor  for,  while 
shifting  winds  may  move 
materials,  they  do  not  tend 
to  pile  up  dunes  of  notable 
heights.  (3)  Then  strong 
winds,  especially  if  they  blow  in  dry  seasons,  are  effective,  other  things 


FIG.  90. 


A  dune  advancing  on  a  forest, 
Indiana. 


110  WIND  WORK  AND  EOLIAN  SOILS 

being  equal.  For  instance  in  oiir  arid  and  subarid  West  and  Southwest, 
southerly  winds  are  strong  and  persistent  in  the  summer.  Sand  drifts 

accumulate  on  the  south  side 
of  dense  hedges  and  on  the 
north  side  of  fences.  Dunes 
form  in  favorable  localities 
along  some  valleys  where 
sand  is  blown  from  uplands 
and  from  the  valleys  of  rivers 
and  lodges  in  a  belt  of  dunes 
on  the  leeward  of  the  valleys. 

FIG.  91.-Tree  planting  to  hold  "Creeping  Not  a11  dunes  are  m°vinS> 
Joe,"  a  traveling  dune,  Mich.  (F.  H.  especially  in  humid  regions, 
Sanford,  Michigan  Agricultural  College.)  where  they  are  often  covered 

by    timber,    shrubs    or    sod, 

but,  with  close  cutting  of  timber,  forest  fires,  too  close  pasturing  or 
otherwise  destroying  the  protective  covering,  the  dunes  are  often 
again  set  in  motion.  Dunes  have  been  successfully  controlled  in 
Europe  by  planting  pine  forests,  which  not  only  hold  the  dunes  but 
afford  revenue  as  well. 

WIND  ABRASION 

This  process  is  especially  prominent  in  dry  regions  with  high  winds. 
In  such  climates  the  sand  is  largely  due  to  disintegration  and,  therefore, 
has  sharp  edges  which  are  so  effective  in  abrasion  that  the  lower  parts 
of  telegraph  poles  are  often  worn  away.  Clear  air,  like  clear  water, 
has  little  abrasive  power  and  becomes  an  effective  abrading  agent  only 
when  it  is  armed  with  sand  and  then  it  has  a  sand  blast  effect  that  is 
seen  in  desert  erosion,  Fig.  92.  The  effectiveness  of  this  sand  blast  is 
often  seen  where  windows  exposed  to  drifting  sand  become  translucent 
in  a  few  months  owing  to  the  sand  abrasion  of  the  glass  surface.  Wind, 
as  a  geological  agent,  however,  is,  for  the  most  part,  important  as  a 
carrier  of  materials  that  have  been  comminuted  by  other  agents,  so  that 
the  load  of  winds  is  for  the  most  part  secured  from  rocks  that  have 
been  broken  up  by  weathering  rather  than  by  wind  work. 

Soil  Blowing. — Wind  work  in  dry  regions  is  a  factor  to  be  reckoned 
with  because  of  the  "  blowing  "  of  soils  by  which  the  soil  from  a  field 
may  be  removed  during  a  single  storm,  Fig.  93.  Corn  rows  that  lie  in 
the  direction  of  prevailing  winds  in  a  dry  region  are  likely  to  be  scoured 


DUNES 


111 


out,  leaving  the  corn  rows  in  relief.  Sandy  soils  of  some  humid  regions 
are  also  subject  to  blowing.  Since  air  is  so  light,  the  winds  are  readily 
checked  by  obstructions  such  as  trees  and  shruds,  which  retard  the 


FIG.  92.— A  wind-abraded  rock  surface,  Arizona.     The  pits  are  largely  due  to  solu- 
tion.    (Gregory,  U.  S.  Geological  Survey.) 


FIG.  93. — "Blowing"  of  soil  due  to  the  destruction  of  protective  vegetation,  Michigan. 
(F.  H.  Sanford,  Michigan  Agricultural  College.) 

movements  of  the  lower  air  and  so  protect  from  wind  erosion;  for  this 
reason,  wind  breaks  of  trees  are  useful  in  some  dry  regions.  Inter- 
lacing roots  bind  the  soil  particles  so  firmly  that,  so  long  as  there  is  a 
fairly  good  sod  or  a  fairly  dense  vegetation  cover,  there  is  little  danger 


112 


WIND  WORK  AND  EOLIAN  SOILS 


of  soil  blowing.  It  has  been  found,  for  example,  that  a  rather  sparse 
growth  of  certain  willows  will  check  soil  blowing  because  these  willows 
have  long,  numerous  underground  stems  and  roots.  Vegetation  also 
keeps  the  soil  moist  so  that  the  soil  grains  are  more  or  less  bound  by 
tenacious  films  of  water.  The  incorporation  of  humus  in  soils  also 
tends  to  check  wind  erosion,  for  humus  tends  to  keep  soils  more  moist. 


THE  LOESS 

The  loess  (German  Ids)  is  a  formation  which  is  undoubtedly  due 
in  large  measure  to  wind  work.  It  is  a  superficial  deposit,  very  impor- 
tant from  a  viewpoint  of  soils  both  because  of  its  wide  exposure  and 
the  fertility  of  its  soils.  With  the  possible  exception  of  alluvial  soils, 
the  loessial  soils  are  probably,  as  a  whole,  the  most  productive  soils  in 
North  America. 

The  loess  is  a  fine-grained,  sitty  material  often  locally  called  "  clay," 
but  it  neither  puddles  nor  holds  water  as  does  clay.  In  color  it  is 

typically  brownish  or  yellowish, 
although  locally  it  may  be  gray 
or  black.  The  most  distinctive 
characteristic  of  loess  is  its  be- 
havior on  exposure  to  weathering 
or  erosion.  Where  other  materials, 
such  as  sand  and  clay,  are  worn 
to  more  or  less  gentle  slopes  the 
loess  stands  in  nearly  vertical 
faces,  and  often  the  vertical  faces 
present  a  rudely  columnar  appear- 
ance as  shown  in  Fig.  94.  Road 
cuts  in  loess  become  small  can- 
yons as  the  soft  material  is  washed 
away  and  old  whiffletree  marks  are 
often  preserved  a  dozen  feet  above 
the  road;  shovel  marks  in  rail- 
road cuts  remain  clear  for  years,  Fig.  95.  The  cause  of  this  peculiar 
characteristic  is  not  fully  understood  . 

Mechanical  Composition. — A  remarkable  feature  of  loess  is  its  gen- 
eral uniformity,  especially  in  mechanical  composition.  It  is  invari- 
ably silty  with  a  content  of  clay  between  20  and  30  per  cent  and  still 


FIG.  94. — Columnar  appearance  of  loess, 
La.  Note  the  steep  face  of  the  loess 
and  the  more  gentle  slopes  of  the 
underlying  clay. 


THE  LOESS 


113 


less  sand.  It  shows  little  or  no  stratification  and  there  is  but  little 
change  in  vertical  section.  A  handful  of  loess  from  Iowa,  for  instance, 
is  but  little  different  from  the  loess  of  Illinois,  Louisiana  or  Europe.  In 
mechanical  composition,  most  of  the  loess  is  a  silt  intermediate  in  size 
between  very  fine  sand  and  clay;  in  other  words,  most  of  the  loess 
particles  are  microscopic.  The  fine  particles  are  usually  somewhat 
angular,  Fig.  96.  The  soils  are  almost  invariably  silt  loams,  as  is  shown 


FIG.  95.  PIG.  96. 

FIG.  95.— Steam  shovel  marks  in  loess  about  15  years  old,  La. 

FIG.  96. — Microphotograph  of  loess  particles.     Enlarged  about  250  diameters. 

by  the  following  table,  which  gives  composite  mechanical  analyses  of 
samples  from  Iowa,  Illinois,  Wisconsin,  Indiana,  Missouri  and  Louisi- 
ana:1 


Gravel. 

Coarse  sand. 

Medium 
sand. 

Fine  sand. 

Very  fine 
sand. 

Silt, 

Clay. 

0.1% 

0.6% 

0.5% 

1.4% 

9.0% 

73.7% 

14.2% 

Mineralogical  Composition. — Chemically  the  loess  is  largely  siliceous 
sand.  Quartz  sand  constitutes  most  of  the  coarser  materials  of  the 
loess  and  even  the  silts  are  largely  of  this  material.  Usually  there  are 
considerable  amounts  of  feldspar,  hornblende  and  other  minerals 

definition  of  soil  classes,  see  page  29. 


114 


WIND  WORK  AND  EOLIAN  SOILS 


present  in  the  silt.     The  following  table  shows  the  minerals  in  the  sands 
and  silts  of  two  important  loessial  soils : 1 

MINERALS  IN  SANDS  AND  SILTS  OF  LOESSIAL  SOILS 


'  MINERALS 
OTHER  THAN 

ABUNDANT  AND  CHARACTERISTIC 
MINERALS  IN 

LESS   ABUNDANT    MINERALS   IN 

QUARTZ    IN 

Sand, 

Silt, 

per 

per 

Sand. 

Silt. 

Sand. 

Silt. 

cent. 

cent. 

Marshall 

12 

15-20 

Orthoclase,  pla- 

Epidote 

Apatite,    mus- 

Hornblende, 

silt  loam 

gioclase,  micro- 

muscovite 

covite,  horn- 

biotite   chlo- 

c 1  i  n  e  ,   oligo- 

blende,  rutile, 

rite,  tourma- 

clase,   a  n  d  e  - 

garnet,      zir- 

line,     ortho- 

sine. 

con,  silliman- 

clase,  zircon, 

ite. 

microline,  sil- 

limanite. 

Marion 

10-12 

12 

Microline,  horn- 

Orthoclase, 

Fluorite,     zir- 

Tourmaline, 

silt  loam 

blende,    ortho- 

hornblende. 

con,    garnet, 

microcline, 

clase. 

tourmaline, 

epidote,      ti- 

plagioclase, 

tanite,    chlo- 

oligoclase, 

rite. 

i 

epidote. 

The  coarser  particles  (sand)  show  a  relatively  percentage  of  minerals 
other  than  quartz  and  both  sand  and  silt  show  about  one-eighth  of 
minerals  other  than  quartz,  a  proportion  rather  higher  than  in  most 
soils.  The  orthoclase,  microcline  and  micas  furnish  potash  and  the 
plagioclase  feldspars,  hornblende,  epidote  and  garnet  furnish  lime. 
The  loess  is  often,  but  not  always,  rich  in  lime  carbonate,  especially  in 
the  upper  Mississippi  Basin,  where  lime  concretions  of  various  shapes 
are  common  and  locally  characteristic  of  the  loess.  Less  common  but 
yet  frequent  are  iron,  and  manganese  concretions. 


ORIGIN  OF  LOESS 

The  Problem  Stated. — The  loess  is  a  superficial  formation  mantling 

many  other  formations.     It  is  relatively  thin,  seldom  exceeding  50  feet 

in  thickness  and  usually  much  thinner.     Several  facts  must  be  explained 

in  order  to  account  for  the  loess.     (1)  It  is  a  material  that  has  been 

1McCaughey  and  Frye,  op.  cit. 


THE  LOESS  115 

transported  for  a  considerable  distance  and  often  deposited  on  materials 
that  are  quite  different  from  the  loess.  (2)  Furthermore,  the  loess  is 
often  resting  on  a  buried  topography  that  has  been  roughened  or 
smoothed  before  the  deposition  of  the  loess.  (3)  Then  the  loess  is 
irregular  in  its  distribution;  in  places  it  is  "thickest  on  divides  and 
again  the  thickest  loess  is  found  in  valleys.  (4)  Fossils  are  scanty  as  a 
rule  and  consist  largely  of  land  or  fresh-water  pond  snails;  in  other 
words,  the  fossils  are  mainly  land  forms. 

The  Possible  Agents. — Obviously  the  loess,  being  a  transported 
material,  must  have  been  carried  by  ice,  water  or  wind.  The  absence 
of  all  characteristic  glacial  features  at  once  excludes  ice  as  the  trans- 
porting agent.  Water  currents,  as  we  see  later,  deposit  their  materials 
in  layers,  say,  for  example,  in  layers  of  sandy  materials  interspersed 
between  layers  of  clay.  In  contrast  to  such  an  arrangement  is  the 
characteristic  uniformity  of  loess  for  scores  of  miles,  a  uniformity 
which  would  be  practically  impossible  for  water  currents  to  produce 
unless  the  loess  were  accumulated  in  lakes  where  currents  are  few  and 
weak.  Lakes  presuppose  basins  inclosed  by  higher  lands  and  such  are 
rarely,  if  ever,  found  in  connection  with  the  loess.  One  other  explana- 
tion might  be  possible;  if  a  stream  should  carry  only  fine  loessial  mate- 
rials, its  deposits  would  necessarily  be  loess.  Such  a  load  would  be 
almost  impossible  except  for  a  small  stream  and,  moreover,  much  of  the 
loess  lies  too  high  for  streams  to  have  deposited  it. 

The  explanation  of  most  loess  as  a  wind  or  eolian  deposit  presents 
the  fewest  difficulties  and  is  generally  accepted  by  geologists,  except 
possibly  for  relatively  small  areas.  As  we  have  seen,  the  winds  carry 
only  fine  materials  and,  therefore,  their  deposits  would  lack  well-marked 
stratification  and  would  be  uniform,  because  wind  load  is  mostly  com- 
posed of  fine  materials. 

Figs.  97  and  170  show  the  close  association  of  the  large  loess  areas 
with  glacial  materials.  Glaciation  will  be  taken  up  in  some  detail  in 
later  chapters  and  at  this  point  it  is  sufficient  to  state  that  not  long  ago, 
geologically  speaking,  much  of  North  America  was  invaded  by  huge 
glaciers  from  the  north.  The  glaciers  swept  up  soil,  boulders  and  many 
kinds  of  materials  and  deposited  them  when  the  ice  melted  back.  The 
ice  also  ground  up  much  of  the  coarse  material  in  transit  and  usually 
left  a  mixture  of  rocks  and  clay  called  "  boulder  clay."  Such  a  surface 
of  more  or  less  incoherent  materials  exposed  by  the  recession  of  the 
glaciers  would  furnish  fine  materials  for  the  winds  to  sweep  up,  especially 
if  the  materials  were  dry.  Furthermore  the  vegetation  presumably  was 


116 


WIND  WORK  AND  EOLIAN  SOILS 


Vieksburg 


scanty  and  winds  would  have  a  free  sweep  and  the  soils,  not  being 
held  by  sod  and  roots,  would  be  easily  swept  away.  The  primary  source 
of  most  loess  in  North  America  and  Europe  is,  therefore,  thought  to  be 
glacial  materials.  On  the  other  hand,  in  China  the  fine  desert  debris, 
broken  up  by  weathering,  furnishes  much  of  the  loessial  materials. 

The  two  tongues  of  loess  along  the  Mississippi  River,  Fig.  98, 
apparently  have  a  somewhat  different  origin.  Of  these,  the  eastern 
belt  is  much  the  wider  and  more  continuous  with  an  average  width  of 
perhaps  forty  miles;  the  western  belt  is  much  narrower  and  less  con- 
tinuous, being  rather  a  series  of 
areas  than  a  continuous  belt,  Fig.  98. 
These  belts  do  not  extend  up  tribu- 
taries as  would  be  expected  if  they 
were  of  alluvial  origin.  It  is  be- 
lieved that  the  Mississippi  loess  is  also 
indirectly  connected  with  the  glacial 
period  in  that  it  is  probably  largely 
derived  from  glacial  deposits.  At 
times  when  the  glaciers  were  melting, 
vast  floods  must  have  come  down 
the  Mississippi  Valley  carrying  and 
depositing  the  fine-grained  glacial 
materials.  As  the  waters  subsided 

when  the  ice  melting  was  less  active,  widely  extended  mud  flats  must 
have  been  exposed  to  winds  which  caught  up  the  dust  and  deposited 
it  on  the  uplands  adjacent  to  the  river  on  both  sides,  the  westerly 
winds  depositing  the  eastern  belt  and  the  easterly  winds  the  western 
belt. 

The  problem  is  only  partially  solved,  however,  when  only  the  agents 
of  transportation  and  deposition,  together  with  the  primary  sources  of 
the  loess,  are  considered.  What  conditions  would  favor  the  accumula- 
tion of  this  fine,  dust-like  material?  Obviously  a  dry  climate  would  be 
a  favorable  condition  and  at  present  loess  is  being  deposited  by  winds 
in  some  dry  areas  in  China;  it  is  not,  however,  at  present  being  deposited 
in  notable  quantities  in  North  America.  It  is  believed  on  good  evidence 
that  a  dry  climate  prevailed  -at  times  during  the  glacial  period  so  that, 
although  the  winds  would  not  have  to  be  necessarily  stronger  or  more 
persistent  than  at  present,  the  conditions  for  their  getting  and  carrying 
dust  loads  were  especially  favorable.  The  angular  shapes  of  the  fine 
loess  particles,  Fig.  96,  are  shapes  that  are  most  easily  carried  by  the 


FIG.  98. — Loess  areas  in  Louisiana 
and  Mississippi. 


THE  LOESS 


117 


winds  for  their  irregularities  cause  the  dust  particles  to  settle  more 
slowly  than  smoother  particles. 

Cooperating  Agents. — While  wind  appears  to  be  the  main  depositing 
agent  of  loess,  other  agents  are  always  contributory.  Weathering 
breaks  up  rocks  so  that  winds  can  carry  the  fine  particles.  Glaciers 
also  grind  up  rock  and  water  from  glaciers  assorts  the  particles,  both 
processes  furnishing  fine  materials.  Streams  are  very  important  in 
that  they  place  their  fine-grained  deposits  so  that  they  are  available  for 
eolian  action.  A  close  association  of  stream  and  wind  work  in  China 
has  been  described  by  Willis.1  Here,  dust  from  interior  deserts  has  been 
swept  into  the  rivers,  carried  down  to  the  lowlands  and  there  deposited 
by  the  rivers  as  sandy  loess.  From  these  lowlands,  the  winds  have 
swept  up  the  loess  and  spread  it  over  the  uplands.  Probably  many  areas 
of  valley  loess  in  North  America  have  had  a  somewhat  similar  origin. 


LOESSIAL  SOILS 

These  soils  are  usually  productive.  They  are  readily  cultivated  and 
are  especially  important  wheat  and  corn  soils  in  the  upper  Mississippi 
Basin.  In  general  these  soils  usually  contain  ample  potash  and  lime  and 
many  have  fair  amounts  of  phosphoric  acid.  The  table  below  gives  a 
general  idea  of  the  plant  food  in  these  soils: 

PLANT  FOODS  IN  POUNDS  PER  ACRE  (ABOUT  7  INCHES)  OF 
LOESSIAL  SOILS1 


Total  nitrogen. 

Total  phosphorus. 

Total  potash. 

Illinois  l  

3,796 

1  140 

33223 

Nebraska  2 

3700 

1  007 

24  110 

Iowa  3  

4,197 

1,422 

32,518 

Louisiana  4  

2,540 

850 

22,084 

l.  123,  University  of  Illinois  Agricultural  Experiment  Station,  by  Cyril  G.  Hopkins  and 
James  S.  Pettit,  1908  (some  non-loessial  soils  are  included). 

2  The  Loess  Soils  of  Nebraska,  by  F.  J.  Alway,  W.  L.  Blish,  R.  M.  Isham,  Soil  Science,  Vol.  1, 
1916. 

'Bull.  150,  Iowa  Agricultural  Experiment  Station,  by  Percy  E.  Brown,  1914. 

*  Data  from  Louisiana  State  Experiment  Station,  I.  Selecter,  Soil  Chemist. 

The  northern  loess  has  been  comparatively  little  leached  and  is 
somewhat  high  in  potash  and  lime.     On  the  other  hand,  the  southern 
loess  materials  have  probably  been  transported  by  the  Mississippi  and 
1  Researches  in  China  by  Bailey  Willis,  Vol.  1,  Carnegie  Institution. 


118 


WIND  WORK  AND  EOLIAN  SOILS 


thus  leached  during  transit  and,  moreover,  they  have  been  subjected 
to  rather  heavy  rainfall  so  that  they  are  somewhat  lower  in  potash 
and  lime.  The  potash  of  the  loess  is  mainly  in  the  form  of  feldspars, 
which  furnish  slowly  available  supplies  of  potash. 

The  structure  and  mechanical  composition  of  loessial  soils  are  espe- 
cially favorable  to  the  movements  of  soil  moisture.  The  soils  are  usually 

porous  enough  to  allow  a  fairly 
ready  downward  movement  of 
gravitational  water,  and  on  the 
other  hand  they  are  fairly  re- 
tentive of  moisture  and  allow 
the  upward  movement  of  capil- 
lary moisture  in  dry  weather. 
The  wide  distribution  of 
loessial  soils  in  the  United 
States  is  indicated  in  Fig.  97, 
where  it  is  seen  that  the  great 
areas  are  in  Illinois,  Iowa  and 
Nebraska.  It  is  a  formation 
distinctively  confined  to  the 
Mississippi  Basin  with  the 
principal  areas  in  the  upper 
Basin  and  with  long  narrow 

FIG.  97.-The  principal  areas  of  loessial  soils  belts  alonS  the  Mississippi 
in  North  America,  (After  Coffey,  U.  S.  fr°m  the  mouth  of  the  Ohio 
Bureau  of  Soils.)  nearly  to  the  mouth  of  the 

Mississippi.  The  obvious  rela- 
tion to  the  Mississippi  and  to  other  rivers  and  the  appearance  of  the 
loess  in  the  bluffs  have  given  the  local  name  "  Bluff  Formation  "  to 
the  loess  and  "  bluff  soils  "  to  the  soils.  The  loess  has  a  wide  distri- 
bution in  China  and  considerable  areas  are  found  in  Europe. 

Three  main  soils  have  been  described  in  North  America.  The  Marshall  silt  loam 
is  a  dark-colored  loessial  prairie  soil,  one  of  the  most  important  corn  soils  of  the  Middle 
West.  The  Memphis  silt  loam  is  typically  a  brownish  soil  which  is  derived  from  the 
loess  that  borders  both  sides  of  the  Mississippi  south  of  the  Ohio.  The  Miami  silt 
loam  is  a  light  brown  soil  which  covers  much  of  southern  Illinois  and  Indiana. 


THE   LOESS  119 

REFERENCES 
Wind  Work 

E.  E.  FREE,  The  Movement  of  Soil  Material  by  Wind,  Bull.  68,  U.  S.  Bureau  of  Soils, 
1911. 

SIR  ARCHIBALD  GEIKIE,  Geology,  Vol.  1,  Macmillan,  1903,  pages  431-446. 

JAMES  GEIKIE,  Earth  Sculpture,  Putnam,  1898,  Chapter  12. 

A.  W.  GRABEAU,  The  Principles  of  Stratigraphy,  Seiler,  1913:  Wind  Erosion,  pages 
52-62;  Modern  Eolian  Deposits,  pages  551-564. 

N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Kept.,  Part  1,  U.  S.  Geological 
Survey,  1890-91;  Wind  Blown  Soils,  pages  326-327. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  Chapter 
on  Eolian  Deposits. 

TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapter  3. 

J.  A.  UDDEN,  Erosion,  Transportation  and  Sedimentation  Performed  by  the  Atmos- 
phere, Journal  of  Geology,  Vol.  2,  pages  318-331. 

U.  S.  Bureau  of  Soils,  Reconnoissance  Soil  Survey  of  Western  Kansas,  1910;  Western 
Nebraska,  1911. 

Loess 

T.  C.  CHAMBERLIN,  Preliminary  Paper  of  the  Driftless  Area  of  the  Upper  Mississippi 
Valley,  6th  Ann.  Rept.  U.  S.  Geological  Survey,  1885,  pages  f  19-322. 

CHAMBERLIN  and  SALISBURY,  Geology,  Vol.  3,  1907,  pages  405-412  (loess  in  connec- 
tion with  glaciation). 

FRANK  LEVERETT,  The  Illinois  Glacial  Lobe,  Monograph,  U.  S.  Geological  Survey, 
1899,  pages  167-177  (American  loess). 

C.  F.  MARBUT  and  J.  E.  LAPHAM,  Soils  of  the  Glacial  and  Losesial  Province,  Bull.  96, 
U.  S.  Bureau  of  Soils,  pages  109-164. 


CHAPTER  VI 
GROUND  WATER 

Ground  water,  as  the  name  implies,  is  the  water  contained  in  the 
earth  as  distinguished  from  the  water  that  runs  on  the  surface.  Its 
presence  is  evidenced  by  the  thousands  of  wells,  springs  and  seepages 
by  which  the  ground  water  reaches  the  surface.  The  ultimate  source 
of  most  of  the  ground  water  is  the  rainfall,  but  the  ground  water  at  a 
given  place  is  not  necessarily  derived  from  local  rainfall  because  there 
is  some  movement  of  ground  water  from  place  to  place.  For  example, 
the  artesian  well  water  of  the  High  Plains  comes  from  the  Rocky  Moun- 
tain area  hundreds  of  miles  to  the  westward.  Much  of  the  ground 
water  ultimately  returns  to  the  surface  by  springs  and  seepages,  some 
enters  the  sea  by  underground  courses,  some  is  drawn  upward  by 
capillary  movement,  some  is  held  in  the  pores  of  rocks  and  a  very  small 
portion  enteis  into  chemical  combination  to  form  hydrated  minerals 
and  so  for  the  time  becomes  fixed. 

The  total  amount  of  ground  water  is  necessarily  undetermined. 
Most  observed  rocks  are  porous  and  contain  some  water,  but  it  is 
believed  that  at  a  depth  of  several  miles  the  rocks  are  compacted  by  the 
pressure  of  the  overlying  materials  so  that  there  is  little  or  no  space  for 
the  accumulation  and  movement  of  ground  water.  To  give  an  idea  of 
the  enormous  quantity  of  ground  water  it  may  be  stated  that  all  esti- 
mates agree  that,  if  the  ground  water  were  brought  to  the  surface,  the 
lands  would  be  covered  to  the  depth  of  scores  of  feet.  The  water 
moves  for  the  most  part  through  the  rock  pores  and  naturally,  in  fine- 
grained rocks  like  shales,  the  movement  is  very  slow.  Sandstones  and 
conglomerates,  because  of  their  greater  pore  spaces,  furnish  the  greatest 
storage  capacity  and  permit  the  most  rapid  movement.  These  rocks, 
therefore,  contain  the  great  stores  of  artesian  water. 

The  Water  Table 

The  water  table  is  the  upper  level  of  the  ground  water  or,  in  other 
words,  the  level  below  which  the  earth's  crust  is  more  or  less  saturated 


THE   WATER  TABLE 


121 


with  ground  water;  the  water  table  is  never  a  definite  surface  like  that 
of  a  lake  but  rather  an  indefinite  zone.  In  swamps  it  may  practically 
coincide  with  the  earth's  surface  while  in  arid  regions  it  may  be 
hundreds  of  feet  underground.  That  the  depth  of  the  ground  water 
varies,  even  in  small  distances,  is  shown  when  a  "  dug  "  well  strikes 
water  at  a  given  depth  and  another  well  near  by  reaches  water 
at  a  different  depth.  When  shallow  wells  "  go  dry  "  the  water  table 
has  sunk  beneath  the  bottoms  of  the  wells.  The  depth  of  the  water 
table  below  the  surface  varies  with  many  factors  including  the  amount 
and  character  of  the  rainfall,  the  kinds  of  materials  such  as  sand, 
clay,  etc.,  and  the  porosity  of  the  materials.  In  general  it  is  not 
level.  It  stands  higher  in  some  materials  than  in  others  and,  more- 
over, it  usually  corresponds  roughly  to  the  surface  above  as  shown  in 
Fig.  99,  although  it  does  not  rise  and  fall  as  sharply  as  the  topography. 
Again  the  level  of  the  water 
table  varies  locally;  for  ex- 
ample, a  heavy  rainfall  will 
raise  it  while  a  long  dry 
spell  will  depress  it.  It  is 
hardly  necessary  to  state  that 
the  depth  of  the  water  table 
is  of  great  economic  impor- 
tance as  the  depth  conditions 
to  some  extent  the  depth  of 
wells.  The  depth  of  the  water 

table  is  also  important  from  an  agricultural  standpoint.  In  very  dry 
seasons  it  descends  so  far  that  the  roots  of  deep-rooting  plants  cannot 
be  supplied  with  moisture  and,  again,  in  wet  seasons  or  in  swampy 
regions  it  may  rise  so  high  that  the  root  zone  of  plants  is  saturated  and 
many  plants  are  injured  in  consequence. 

There  is  naturally  no  sharp  distinction  between  soil  water  and 
ground  water  although,  of  course,  the  former  is  included  in  the  latter. 
Soil  water  is  an  extremely  small  portion  of  the  ground  water  but,  from 
an  agricultural  point  of  view,  it  is  an  extremely  important  part.  We 
shall  first  consider  the  general  features  of  ground  water  and  then  apply 
the  principles  to  soil  water. 


FIG.  99. — Diagram  to  show  a  common  rela- 
tion between  topography  and  ground  water 
(dotted).  Springs  (S)  occur  where  the 
water  table  is  reached  in  valleys. 


122  GROUND  WATER 


Ground  Water  Movements 

These  movements  are,  in  general,  very  slow  as  compared  with 
the  movements  of  surface  waters.  First  there  is  the  downward  gravi- 
tational movement  by  which  the  water  descends  to  the  water  table. 
This  movement  is  approximately  in  a  vertical  direction  although, 
owing  to  differences  in  rocks,  the  movements  may  be  in  a  combination 
of  downward  and  lateral  directions.  Then  there  is  the  "underflow," 
which  is  common  along  streams  that  are  underlain  by  gravels  and  sands. 
For  example,  some  portions  of  the  Arkansas  River  may  be  dry  but 
beneath  the  surface  in  the  sands  and  gravels  there  is  always  a  slow 
movement  down  stream.  In  some  rocks,  especially  limestones,  there 
are  large  openings  through  which  the  water  movement  is  relatively 
rapid  and  this  is  also  true  of  movements  in  crevices  and  joints,  but  for 
most  of  the  ground-water  movement,  the  term  percolation  would  be 
most  appropriate. 


Work  of  Ground  Water 

This  may  be  considered  under  two  heads,  the  mechanical  and  the 
chemical  work.  Because  of  the  slow  movement  of  ground  water  its 
mechanical  work  is  but  of  slight  importance  except  in  soils,  and  this 
topic  will  be  considered  under  soil  water.  The  chemical  work  of  ground 
water  may  be  considered  under  two  heads,  solution  and  deposition. 
These  and  other  topics  related  to  ground  water  have  been  considered 
under  weathering,  but  the  importance  of  the  process  will  justify  some 
repetition  here. 

Solution. — The  universality  of  this  process  is  proved  by  the  fact 
that  practically  no  stream  or  well  waters  are  free  from  materials  in 
solution.  Lime  compounds,  as  a  rule,  are  the  most  common  materials 
dissolved  in  natural  waters,  a  fact  which  is  proved  by  the  common  lime 
coatings  in  kettles  and  boilers.  Solution  is  promoted  by  several  factors. 
(1)  Practically  all  ground  water  contains  carbonic  acid  and  probably 
other  acids  in  solution  so  that  the  soluble  minerals  in  rocks  and  soils 
are  in  time  dissolved.  (2)  It  is  a  matter  of  common  observation  that 
the  solvent  action  of  water  is  greatly  increased  by  heat.  Some  deep 
mineral  deposits  are  thought  first  to  have  been  dissolved  by  hot  waters, 
and  subsequently  deposited,  and  some  hot  springs  are  now  depositing 
not  only  soluble  minerals  but  silica,  which  is  ordinarily  insoluble  in 


WORK  OF  GROUND  WATER 


123 


FIG.  100. — Calcareous  tufa,   a  hot   spring 
deposit,  Cal.     (U.  S.  Geological  Survey.) 


water,  Fig.  100.  (3)  An  increase  in  pressure  increases  the  solvent  power 
of  water  and,  of  course,  (4)  a  decrease  in  pressure  lessens  the  ability  of 
water  to  hold  substances  in  solution.  (5)  Finally,  other  things  being 
equal,  it  is  evident  that  time  is  an  important  factor  in  solution.  The 
longer  ground  water  is  in  con- 
tact with  a  substance  the  greater 
the  solvent  action,  and  for  this 
reason  slowly  moving  ground 
water  is  much  more  effective, 
volume  for  volume,  than  the 
waters  of  streams. 

Caverns  and  Sink  Holes. — 
Perhaps  the  most  striking 
features  due  to  the  work  of 
solution  are  the  caverns  which 
are  so  frequent  and  large  in 
many  limestone  regions  because 

limestones,  as  has  been  seen,  are  especially  soluble  in  ground  water. 
The  water  percolating  along  joints  or  bedding  planes  dissolves  the  rock 
and  constantly  enlarges  its  channels  until  a  system  of  galleries  and 
rooms  is  formed,  often  at  different  levels.  Shaler  estimates  that,  in 
the  great  Mammoth  Cave  of  Kentucky,  there  are  probably  more  than 
200  miles  of  fairly  continuous  openings  which,  at  times,  enlarge  into 

spacious  chambers  and  again 
are  so  narrow  that  one  has 
difficulty  in  passing  through 
them.  Another  common  ac- 
companiment of  caverns  and 
other  solution  work. are  sink 
holes,  round  depressions  in  the 
bottoms  of  which  there  are 
usually  passageways  by  which 
surface  waters  escape  down- 
wards. Such  a  sink  hole  is 

shown  in  Fig.  101.  These  underground  passageways  often  become 
obstructed,  either  accidentally  or  purposely,  and  the  sink  holes  become 
ponds  which  are  often  used  as  stock  ponds.  Where  there  are  many 
sink  holes  they  produce  a  peculiar  characteristic  "  billowy " 
topography. 


FIG.  101. — A  sink  hole,  Tennessee. 
Bureau  of  Soils.) 


(U.  S. 


124 


GROUND  WATER 


Deposition  by  Ground  Water 

This  process  is  the  factor  to  which  we  are  indebted  for  most  of  the 
important  deposits  of  iron,  zinc,  lead,  copper  and  some  gold  and  silver . 
deposits.  The  phenomena  of  deposition  by  deep-seated  waters  are  very 
difficult  to  explain  completely  but  it  may  be  said  that,  whenever  the 
ground  water  becomes  saturated  with  a  given  mineral,  that  mineral 
will  be  deposited.  Probably  first  in  importance  among  the  factors 
of  deposition  is  a  (1)  decrease  in  temperature  by  which  the  water  loses 
its  solvent  powers.  (2)  A  decrease  in  pressure  also  lowers  the  solvent 
power  of  water,  as  when  deep  waters  rise  into  regions  of  less  pressure. 
(3)  Finally  there  are  chemical  exchanges  between  different  solutions, 
as  when  scrap  iron  is  thrown  into  water  containing  copper  and  the  copper 

slowly  replaces  the  iron.  An  interesting 
example  of  substitution  is  when  wood  and 
other  organic  materials  change  to  stone, 
a  process  termed  petrefaction.  The  par- 
ticles of  wood  decay  and  are  replaced 
by  silica  or  other  substances  which  are 
in  solution  in  the  ground  water. 

Mineral  veins  are  common  occur- 
rences in  workable  ores.  The  ground 
water  passing  through  crevices  or  zones 
of  weakness  deposits  various  minerals 
often  in  roughly  arranged  bands.  Usually 
the  water  deposits  not  only  valuable 
minerals  but  also  other  minerals  which 
are  regarded  as  impurities,  of  which  the 

principal  ones  are  quartz  and  calcite.  Many  veins  are  composed 
entirely  of  these  minerals  or  other  minerals  of  little  or  no  commercial 
value. 


FIG.  102. — Microphotograph  of 
chert.  Ground  water  has  de- 
posited the  minute  layers. 
(U.  S.  Geological  Survey.) 


SOIL  WATER 

This  is  the  ground  water  of  the  soil,  constituting,  of  course,  the  upper 
part  of  the  great  mass  of  ground  water.  The  processes  are  almost  alike 
in  the  soil  water  and  in  the  deeper  ground  water,  but  not  all  the  factors 
are  equally  important.  (1)  In  general,  soil  water  contains  more  oxygen 
and  carbon  dioxide,  (2)  it  has  less  pressure,  (3)  is  subject  to  more  active 
evaporation  and  (4)  the  downward  movement  of  the  soil  water  is  in 


SOIL  WATER 


125 


general  much  faster  than  in  the  deeper  ground  water.  (5)  Furthermore, 
the  mechanical  work  of  soil  water  is  of  great  agricultural  importance 
while  that  of  the  deep  ground  water  is  almost  negligible.  Hydration, 
oxidation,  and  solution  vary  with  so  many  factors  that  comparisons  of 
their  effectiveness  in  the  soil  water  and  deeper  ground  water  are  hardly 
possible. 

Movements. — The  familiar  downward  or  gravitational  water  results 
from  the  sinking  of  rainfall  into  the  ground.  The  rapidity  of  down- 
ward movement  is  obviously  affected  by  the  porosity  of  the  soil,  for  it  is 
a  matter  of  common  observation  that  rain  will  sink  into  sandy  soils 
more  rapidly  than  into  clay  or  silty  soils  and  more  rapidly  into  silt  loams 
than  into  clays.  A  sudden,  brief  heavy  rain  will  penetrate  less  than 
the  same  amount  of  water  falling  for  a  longer  time,  since  it  takes  time 
for  water  to  expel  the  soil  air  and  work  its  way  downward  among  the 
soil  particles.  The  downward  movement  is  hastened  by  soil  cracks, 
roots,  root  paths  and  the  holes  of  burrowing  animals. 

Capillary  Water. — The  rise  of  oil  in  a  lamp  wick  and  the  fact  that  a 
towel  hanging  over  the  side  of  a  basin  will  in  time  drain  the  basin  are 
familiar  llustrations  of  capillarity.  Similar  capillary  movements  occur 
in  soils  and  are  important.  This  movement  takes  place  in  all  directions, 
the  direction  of  movement  being  from  areas  of  more  soil  moisture  to  those 
of  less  soil  moisture.  Capillarity  thus  aids  the  downward  movement 
and  promotes  drainage.  The  upward  movement  brings  moisture  to 
soil  and  subsoil  and  is  beneficial  to  crops  in  dry  weather.  When  the 

capillary  moisture  reaches  the 
surface,  it  is,  of  course,  evapor- 
ated; soil  mulches  are  produced 
to  check  this  evaporation. 

All  soils  do  not  favor  capillary 
movement  equally  well.     Fine- 
textured  soils,  like  very  fine  sands, 
sandy  loams,  loams  and  some  silt 
loams  favor  a  rapid  movement 
of  capillary  water.    Coarse  sandy 
soils  and  very  heavy  clay  soils 
FIG.   103. — The  water  table  (heavy  line)     are  less  favorable  for  this  move- 
in  the  soil  to  the  right  is  depressed  by     ment.     In  general,  a  texture  be- 
coarse  gravel.     (After  Hilgard.)  tween  ft  coarse  sand  and  a  heavy 

clay  is  favorable  for  capillarity 
or,  in  other  words,  small   soil  grains,  unless  they  are  very  small,  are 


126  GROUND  WATER 

most  favorable  to  capillary  action.  Sometimes  a  coarse  gravel 
below  the  subsoil  will  cut  off  capillary  moisture  and  cause  a  "  bald 
spot "  in  a  dry  season,  Fig.  103.  While  the  upward  movement 
of  capillary  water  has  been  emphasized,  it  should  be  remembered  that 
it  moves  in  all  directions.  The  movement  is  very  slow  and  incon- 
spicuous, but  its  importance  is  shown  by  King's  estimate  that,  under 
most  favorable  conditions,  if  the  movement  were  continuous  and  the 
supply  sufficient,  over  63  inches  of  water  could  be  delivered  to  the 
earth's  surface  in  a  year,  an  amount  greater  than  the  annual  rainfall 
in  most  regions. 

Mechanical  Work  of  Soil  Water. — In  humid  regions  the  subsoils 
are  usually  heavier  in  texture  than  the  soils.  A  sandy  soil  is  likely  to 
have  a  sandy  loam  or  a  sandy  clay  subsoil;  a  sandy  loam  often  has  a 
subsoil  of  sandy  clay  and  a  silt  loam  is  often  underlain  by  a  silty  clay 
loam  or  silty  clay.  Even  a  clay  soil  usually  has  a  heavier  subsoil, 
although  commonly  there  is  not  so  much  difference  between  soil  and  sub- 
soil in  clay  soils.  The  main  reason  for  this  difference  between  soil  and 
subsoil  in  humid  regions  is  that  the  gravitational  water  in  its  relatively 
rapid  descent  carries  the  finer  materials  such  as  silt  and  clay  which  are 
found  in  nearly  all  soils  into  the  subsoils.  Here  the  downward  move- 
ment is  checked,  the  carrying  power  of  the  descending  ground  water  is 
diminished  and  the  finer  particles  are  deposited  in  the  pores  between 

,the  larger  particles  of  the  sub- 
soil.      Fig.    104    illustrates   the 


,,LTYV:C*XY;:WAV- *f-     -:  .V^.NOV  ^AW  ,  ;         occurrences  on  subsoils  in  Kansas 

and    Louisiana    where    both    a 
FIG.  104. — Soil   and   subsoil   in    (1)  loess;  heavy  soil    and   a   light  soil   are 
(2)  in  sandy  loam.  underlain    by    heavier    subsoils. 

Naturally  in    arid  regions  there 

is  a  weak  gravitational  movement  and  the  subsoils  are  not  so  well 
developed  so  that  there  is  little  difference  between  soil  and  subsoil. 

Chemical  Work  and  Soil  Water. — This  work  has  been  in  part  dis- 
cussed under  the  topic  of  "Weathering." 

Oxidation. — Descending  gravitational  water  naturally  carries  con- 
siderable oxygen  and  many  porous  soils  are  well  oxidized  and  have 
reddish  or  brownish  colors  due  to  coatings  of  iron  oxides  on  the  soil 
grains.  Subsoils  show  much  variation  in  oxidation.  Porous  subsoils 
are  likely  to  be  reddish  and  well  oxidized;  heavy  silty  clay  soils  may 
be  underlain  by  subsoils  that  show  the  reddish  and  brownish  colors  due 
to  oxidation  in  their  upper  portions,  while  the  lower  subsoil  may  be 


SOIL  WATER  127 

oxidized  in  spots,  and  still  further  down  the  deep  subsoil  may  be  bluish 
or  grayish  and  practically  unoxidized.  In  many  cases  red  or  brown 
subsoils  are  favorable  indications  when  these  colors  are  due  to  active 
oxidation  since  they  indicate  good  aeration.  Here  as  elsewhere  hydra- 
tion  commonly  accompanies  oxidation,  and  limonite,  the  hydrated  iron 
(Fe2O3-3H2O),  is  often  apparent  in  soils  because  of  its  yellowish  colors. 

Carbonation  is  especially  active  in  soils,  both  because  the  supply  of 
carbon  dioxide  is  derived  from  the  decaying  vegetable  matter  in  soils, 
and  the  rains  carry  large  amounts  from  the  air  into  the  soils.  Car- 
bonic acid  is  a  weak  acid  and  does  not  work  actively,  but  in  humid 
climates  an  enormous  amount  is  carried  down  by  gravitational  water 
and  this  quantity,  in  some  degree,  compensates  for  the  weakness  of  the 
acid  because  of  its  dilution.  The  carbonates  are  in  general  fairly  soluble, 
especially  if  there  is  an  excess  of  carbonic  acid  in  the  solution.  The 
soluble  lime  and  magnesian  carbonates  are  common  in  many  soils,  and 
the  iron  carbonates,  while  less  soluble,  are  also  found  in  many  soil 
solutions. 

Solution  is  an  important  and  widespread  work  of  soil  waters  which 
always  contain  some  acids.  The  gravitational  water  moves  relatively 
rapidly  through  the  soil  after  each  wetting  of  the  surface  and  generally 
less  rapidly  through  the  subsoil,  so  that  the  more  soluble  minerals  are 
leached  from  the  soil  and  to  a  less  extent  from  the  subsoil.  This  long- 
continued  leaching  is  especially  effective  on  many  sandy  soils.  It  is 
often  shown  in  sags  where  the  water  accumulates  and  the  underlying 
soil  is  gray  from  leaching.  The  red  soils  of  the  Orangeburg  series  in 
the  southern  Coastal  Plain  usually  have  a  few  inches  of  gray  leached 
soils  overlying  brilliant  red  subsoils.  The  capillary  water,  bulk  for 
bulk,  has  a  greater  solvent  effect  than  the  gravitational  water,  for  it 
moves  much  more  slowly  and,  therefore,  is  longer  in  contact  with  the 
soil  particles.  Moreover,  when  exposed  to  the  action  of  capillary 
water,  each  soil  particle  is  closely  and  tenaciously  surrounded  by  a  film 
of  water  which  dissolves  soluble  materials  from  the  surface.  This  water 
film  is  very  thin  and  a  small  amount  of  water  covers  many  square  feet 
of  soil  particles,  so  that  total  solvent  effect  of  this  inconspicuous  solvent 
is,  therefore,  very  important* 

Deposition. — A  casual  examination  of  many  soils  and  subsoils  will 
show  evidences  of  deposition  by  soil  water  in  the  shape  of  red,  brown, 
black  or  white  concretions  (" gravel")  or  in  some  cases  in  the  formation 
of  "hardpan."  Furthermore,  a  microscopic  examination  is  likely  to 
disclose  concretions  in  almost  any  soil.  These  concretions,  although 


128  GROUND  WATER 

often  called  gravel,  are  not  to  be  confused  with  actual  gravels  which 
have  been  deposited  by  water  or  weathered  from  rocks. 

The  gravitational  water  doubtless  deposits  some  of  its  dissolved 
minerals  in  the  subsoil  especially  if,  as  is  often  the  case,  the  subsoil 
is  fine  textured  and  so  retards  the  descent  of  the  soil  solution.  But  it 
is  probable  that  most  of  the  dissolved  materials  in  the  ground  water 
are  carried  below  the  subsoil  and  some  are  there  deposited  at  varying 
depths.  On  the  other  hand,  the  ascending  capillary  water  must  deposit 
much  of  dissolved  materials  in  the  subsoils  for  at  least  two  reasons. 
As  the  ascending  capillary  water  reaches  the  subsoil,  the  pressure  is 
lessened  and  the  water  evaporates.  Furthermore  it  will  be  remembered 
that,  with  release  in  pressure,  the  ground  water  tends  to  deposit  its 
solution  load.  Lime  carbonate  (calcite)  in  the  soil  affords  an  example. 
The  carbonic  acid  in  the  ground  water  changes  the  lime  carbonate  into 
the  soluble  bicarbonate  according  to  a  reaction  which  may  be  represented 
by  the  following  equation: 

Calcite     and    Carbon  dioxide       and      Water     produce       Lime  bicarbonate 
CaCO3      +  CO2  +        H2O  Ca(HCOu)2 

The  lime  bicarbonate  is  somewhat  unstable  and,  when  warmed  or 
the  pressure  released,  it  easily  reverts  to  the  relatively  insoluble  lime 

carbonate.  Now,  when  the  soil  solu- 
tion reaches  a  more  porous  material 
or  enters  cavities,  a  portion  of  the 


FIG.  105'.-Diagram  to  illustrate  the  carbon  dioxide  escapes  and  the  lime 
frequent  occurrence  of  hardpan  carbonate  is  deposited  as  concretions 
and  concretions  between  soil  and  or  as  a  coating  on  the  soil  grains,  or 
subsoil.  as  an  irregular  stratum  or  hardpan 

and  since  the  subsoil  is  usually  less 

porous  than  the  soil,  it  follows  that  deposits  are  often  found  near 
the  merging  of  soil  and  subsoil  as  shown  in  Fig.  105. 

Oxidation  usually  promotes  deposition  in  soils  because  the  oxides 
formed  are  relatively  insoluble  and  the  same  is  true  of  the  common 
hydrates  (combinations  with  water)  in  soils.  The  iron  oxide,  hema- 
tite (Fe2O2)  and  the  hydrated  iron  oxide,  limonite  (Fe2Os  •  3H20),  are 
very  common  in  soils,  as  reddis'h  and  yellowish  coating  on  soil  grains  and 
as  concretions.  Black  concretions,  composed  mostly  of  iron  and  man- 
ganese oxides,  are  not  uncommon,  especially  in  heavy  soils.  Sometimes 
these  concretions  are  so  numerous  as  to  form  a  hardpan  or  a  kind  of  bog 
iron  ore  or  "  ironstone  "  and  the  same  feature  is  often  seen  in  "  lime 


SOIL  WATER  129 

hardpans."  It  is  probable  that  many  of  these  concretions  were  carried 
up  as  soluble  compounds  by  capillary  water  to  the  soil  and  upper  sub- 
soil where  oxygen  is  plentiful  and  there  changed  to  less  soluble  oxides 
and  deposited.  However,  there  are  many  complex  reactions  in  the 
soil  solution  which  are  as  yet  imperfectly  understood  and  which  offer  a 
valuable  and  fruitful  field  for  further  investigation.  Recent  studies 
have  indicated  that  it  is  probable  that  concretions  have  practical  interest 
in  that  many  of  them  seem  to  contain  compounds  of  phosphorus  which 
are  so  slowly  soluble  that  little,  if  any,  of  their  phosphoric  acid  is  avail- 
able for  crops.  Finally,  it  should  not  be  forgotten  that  roots  bring 
up  soluble  materials  to  the  soil  and  subsoil  and  some  of  these  materials 
are  again  put  into  circulation  in  gravitational  and  capillary  waters. 


FIG.  106. — Soil  concretions.     The  three  on  the  left  are  of  lime,  on  the  right,  mostly 

of  iron  compounds. 

Hardpan  is  an  indefinite  term  usually  denoting  a  relatively  hard 
or  impervious  layer  beneath  the  soil  or  in  ^he  subsoil.  In  regions  of 
scanty  rainfall  hardpan  is  very  commonly  of  lime  or  magnesium  car- 
bonate where  it  may  be  due  to  scanty  gravitational  water  which  carries 
the  materials  downward  to  certain  depths  where  they  are  deposited,  or 
it  may  be  due  to  rising  capillary  water.  In  many  cases  the  hardpan  is 
probably  due  to  both  processes.  When  irrigation  water  is  used  too 
plentifully  both  of  the  above-mentioned  processes  are  intensified  and 
it  not  infrequently  happens  that  hardpans  form  beneath  irrigated 
orchards  and  cause  them  to  be  abandoned.  In  humid  regions  hard- 
pans often  form  in  the  subsoil  where  the  fine  materials  of  the  subsoil  are 
cemented  by  lime  carbonate  or  by  iron  oxides. 

"  Alkali." — One  of  the  most  important  agricultural  problems  in 
dry  countries  is  the  deposition  of  alkali  in  soils,  the  "rise  of  the  sub  " 
as  it  is  often  termed.  "Alkali"  is  a  general  term  including  several 
soluble  salts,  among  them  being  sodium  sulphate  (Na2SC>4),  NaCl 
(''salt"),  magnesium  sulphate  (MgSC>4)  and  sodium  carbonate  (Na2COa) 
or  "  black  alkali."  The  black  alkali  is  the  most  dreaded;  it  may  be 
white  in  color,  and  its  dark  color  is  due  to  its  action  on  humus.  Very 
often  the  so-called  alkali  includes  many  or  all  of  these  compounds, 
which  are  injurious  to  crops. 


130  GROUND  WATER 

The  presence  of  these  salts  is  a  notable  work  of  rising  capillary 
water.  The  soils  and  subsoils  are  but  little  leached  and  there  is,  there- 
fore, an  abundance  of  soluble  minerals.  Much  of  the  gravitational 
water  descends  for  a  relatively  short  distance  and  much  of  it  rises  by 
capillarity,  a  rise  doubtless  accelerated  by  the  rapid  evaporation  at 
the  surface  of  the  soil.  When  the  capillary  water  reaches  the  soil  or 
the  surface  it  evaporates  and,  of  course,  leaves  its  dissolved  compounds. 
It  is  a  common  experience  that,  when  an  excess  of  irrigation  water  is 
applied,  the  water  table  rises  towards  the  surface,  more  capillary  water 
therefore  passes  through  the  soils  and  the  lands  are  temporarily  ruined 


FIG.  107. — Patches  of  alkali  in  alfalfa.  The  soil  was  formerly  productive,  but  alkali 
has  been  brought  up  by  capillary  water  derived  from  irrigation  water,  Arizona. 
(U.  S.  Bureau  of  Soils.) 

for  most  crops.  However,  when  heavy  applications  of  water  are  made 
and  this  water  is  removed  by  subsoil  drainage,  the  salts  may  be  washed 
from  the  soils.  Probably  many  soils  in  humid  regions  would  become 
alkaline  if  the  rainfall  were  to  become  scanty  for  long  periods. 

REFERENCES 
Soil  Water 

E.  W.  HILGARD,  Soils,  Macmillan,  1911,  Soil  and  Subsoil,  Chapters  8-9;  Soil  Water, 

Chapters  12,  13,  14;  Soils  of  Humid  and  Arid  Regions,  Chapters  20,  21;  Alkali 

Soils,  Chapters  22,  23. 

ISAIAH  BOWMAN,  Forest  Physiography,  Wiley  &  Sons,  1911,  Chapter  3. 
CAMERON  and  BELL,  The  Mineral  Constituents  of  the  Soil  Solution,  Bull.  30.  U.  S. 

Bureau  of  Soils,  1905. 

LYON,  FIPPIN  and  BUCKMAN,  Soils,  Macmillan,  1916,  Chapter  11. 
WHITNEY  and  CAMERON,  Investigations  is  Soil  Fertility,  Bull.  23,  U.  S.  Bureau  of 

Soils,  1904,  pages  5-23. 


WELLS  AND  SPRINGS  131 

WELLS  AND  SPRINGS 

Springs  occur  when  the  ground  water  escapes  at  the  surface;  the 
flow  may  be  strong  or  merely  a  seepage.  Most  commonly  springs  occur 
in  valleys  where  the  water  table  readily  reaches  the  surface,  Fig.  99. 
There  is  usually  a  relatively  impervious  layer  like  clay  or  a  close-grained 
rock  which  is  overlain  by  porous  materials  like  sand.  The  water  sinking 
through  the  sand  flows  along  the  clay  to  the  point  of  escape  at  the 
spring.  Sandstone  is  so  porous  that  springs  are  common  in  a  "  sand- 
stone country."  Limestone  is  likely  to  contain  openings  along  which 
water  flows  sometimes  as  an  underground  stream  which,  when  it  emerges, 
makes  a  large  spring.  Some  springs  flow  along  joints  or  faults  until 
they  emerge  at  the  surface. 

Wells. — Ordinary  shallow  wells  are  sunk  until  they  reach  the  water 
table.  Sometimes  wells  are  dug  or  driven  into  rock  until  a  supply  of 
"  living  water  "  is  found  in  a  porous  stratum  usually  of  sand  or  sand- 
stone. Artesian  wells  are  those  from  which  water  flows  or  in  which 
water  rises  from  the  bottom.  In  other  words,  these  wells  tap  an  under- 
ground supply  which  is  under  "head,"  that  is,  the  source  of  the  water  is 
higher  than  the  bottom  of  the  well.  In  general  there  are  two  usual 
necessary  conditions  for  artesian  wells.  There  must  be  a  porous  stratum 
to  hold  the  water  and  allow  it  to  flow  and. this  porous  stratum  must  be 
enclosed  by  relatively  impervious  materials  to  confine  the  water.  Again 
the  stratum  must  be  dipping  so  as  to  furnish  a  "  head  "  for  the  water. 
The  outcrop  of  the  water-bearing  stratum  is  termed  the  catchment  area 
because  the  rain  and  snow  falling  here  furnish  the  supply  which  enters 
the  stratum  and  is  available  for  artesian  wells.  Other  things  being 
equal,  the  lower  the  dip  the 

AT 

larger  the  catchment  area,  as 
is  illustrated  in  Fig.  51. 

Artesian  wells  have  become 

very    important     sources    of 

T       ,     ,,     ,,         ...  FIG.  108. — Generalized  diagram  showing  the 

water  supply,  both  tor  cities  ,  •.                          7-3     •»  •  ti_  *? 

^  J '           .  catchment  area  east  of  the  Rocky  Moun- 

and  for  rural  districts.     It  is         tains  from  which  the  sandstones  (dotted) 

extremely  fortunate  that  our         carry  the  underground  water  beneath  the 

dry  plains   in   the   West   are         Plains. 

underlain    by    structure    and 

materials  that  are  favorable  for  artesian  wells.     Such   structure   and 

arrangement  are  seen  in  Fig.  108,  where  the  important  water-bearing 

Dakota  sandstone  rises  steeply  near  the  eastern  front  of  the  Rocky 


132  GROUND  WATER 

Mountains  and  there  forms  an  extensive  catchment  area.  The  rain 
and  snow  in  this  catchment  area  sink  into  the  porous  sandstones  and 
slowly  flow  underneath  the  plains  to  the  eastward.  The  structure  of 
the  Coastal  Plain  in  the  United  States  is  also  very  favorable  for 
artesian  water,  which  is  being  extensively  utilized,  although  most  of 
this  area  has  an  ample  rainfall.  A  little  reflection  will  show  that  a 
frequently  stated  assertion  that  "  the  underground  waters  are  prac- 
tically inexhaustible  "  is  untenable.  At  most  no  more  water  can  enter 
the  rocks  than  falls  on  the  catchment  area  and  not  all  of  this  water  is 
available  for  artesian  wells. 

REFERENCES 

T.  C.  CHAMBERLIN,  Requisite  and  Qualifying  Conditions  of  Artesian  Wells,  5th  Ann. 

Rept.,  U.  S.  Geological  Survey,  1885,  pages  125-173. 
CHAMBERLIN  and  SALISBURY,  Geology,  Holt,  1904,  Vol.  1,  Chapter  4. 
M.  L.  FULLER,  Controlling  Factors  of  Artesian  Flows,  Bull.  319,  U.  S.  Geological 

Survey,  1908,  pages  1-44. 

M.  L.  FULLER,  Domestic  Water  Supply  for  the  Farm,  Wiley  &  Sons,  1912. 
JAMES  GEIKIE,  Earth  Sculpture,  Putrmm,  1898,  Chapter  13. 

E.  O.  HOVEY,  Celebrated  American  Caverns,  Cincinnati,  1896. 

F.  H.  KING,  Movements  of  Ground  Waiter,  19th  Ann.  Rept.,  Part  2,  U.  S.  Geological 

Survey,  1898,  pages  71-100. 

RIES  and  WATSON,  Engineering  Geology,  Wiley  &  Sons,  1914,  Chapter  6. 
R.  D.  SALISBURY,  Physiography,  Holt,  1907,  Chapter  3. 
N.  S.  SHALER,  Caverns  and  Cavern  Life,  Aspects  of  the  Earth,  Scribners,  1889, 

pages  98-142. 
C.  R.  VAN  HISE,  A  Treatise  on  Metamorphism,  pages  63-81  ( Aqueous  Solutions  of 

Ground  Water). 


CHAPTER  VII 
STREAMS   AND   THEIR   WORK:   ALLUVIAL   SOILS 

IN  practically  all  habitable  regions  the  topography  is  being  shaped 
by  stream  work  and,  indeed,  this  must  have  been  true  since  the  begin- 
ning of  geological  time.  Most  hills  and  mountains,  nearly  all  valleys 
and  in  fact  most  surface  features  show  in  some  measure  the  work  of 
streams.  Enormous  areas  of  productive  soils  are  due  to  stream  work. 

Sources  of  Streams. — Rainfall  is  the  primary  source  of  all  streams, 
although  there  are  apparent  exceptions  where  a  river  like  the  Colorado 
flows  through  an  arid  region,  but,  in  this  instance,  the  river's  head 
waters  are  in  a  more  humid  region.  Not  all  the  rain  escapes  through 
streams,  for  a  portion  sinks  into  the  ground  (the  cut-off),  another  por- 
tion runs  off  mostly  in  streams  (the  run-off),  while  still  another  portion 
escapes  by  evaporation.  It  has  been  estimated  that  approximately 
one-fourth  the  rainfall  of  the  world  escapes  by  run-off  and  the  removal 
of  forests  and  the  improvement  of  drainage  by  tile  and  ditches  is  arti- 
ficially increasing  the  run-off.  The  ground  water  also  contributes  to 
streams  when  it  sinks  into  the  ground  and  emerges  as  springs  or  as  the 
less  noticeable  seepages,  but  the  amount  of  ground-water  contribution 
to  streams  is  likely -to  be  underestimated  because  it  is  inconspicuous. 
Streams  are  permanent  when  fed  by  ground  water  and  intermittent 
wnen  fed  only  by  run-off;  they  are  often  permanent  in  their  lower 
courses  where  the  water  table  (page  120)  is  reached  and  intermittent 
above  this  point. 

Stream  Organization. — On  a  level  slope  with  an  equal  precipitation 
over  the  surface,  the  run-off  would  consist  of  a  sheet  of  water  covering 
the  entire  surface,  a  condition  of  sheet  flow  found  in  a  few  localities. 
But  such  conditions  are  rare  since  the  precipitation  is  unlikely  to  be 
uniform,  even  in  small  areas,  and  natural  drainage  lines  are  likely  to  be 
found  even  on  apparently  level  surfaces.  Hence  it  is  the  most  of 
the  run-off  is  accomplished  in  various  degrees  of  efficiency  by  streams. 
The  primary  purposes  of  streams  from  a  geological  point  of  view  are 
(1)  promoting  the  run-off  and  (2)  carrying  the  land  debris  to  the  sea. 

133 


134  STREAMS  AND  THEIR  WORK:  ALLUVIAL  SOILS 

The  velocity  of  a  stream  depends  (1)  mainly  on  the  slope  of  its 
bed,  and  since  most  streams  have  steeper  slopes  in  their  upper  portions, 
it  is  there  that  they  are  usually  swiftest.  (2)  The  velocity  also  varies 
according  to  the  volume;  streams  flow  fastest  in  times  of  high  water. 
(3)  Somewhat  less  obvious  is  the  fact  that  stream  velocity  varies  with 
the  amount  of  load  that  a  stream  is  carrying.  If  a  quantity  of  loose 
material  like  sand  or  sawdust,  for  example,  is  thrown  into  a  stream,  the 
current  becomes  slower.  This  last  factor,  while  not  easily  observed,  is 
believed  to  be  important  in  changing  some  streams  from  depositing  to 
carrying  streams  and  vice  versa.  (4)  Other  things  being  equal,  a 
stream  flows  faster  in  a  straight  channel  than  in  a  crooked  one  and 
(5)  in  a  smooth  than  in  a  rugged  one.  (6)  Finally  a  stream  flows  less 
rapidly  at  the  top  and  the  bottom  because  of  the  friction  of  the  air  and 
the  bottom,  respectively,  and  (7)  less  rapidly  at  the  margins  because 
the  shallow  water  there  also  meets  with  more  friction. 

STREAM   WORK 

STREAM  EROSION 

Introductory. — Streams  are  incessantly  wearing  down  the  land  and 
transporting  the  debris  to  the  sea.  Small  streams  during  and  after 
rains  become  muddy  because  of  their  traveling  load  of  fine  sand,  silt 
and  clay,  and  large  rivers  are  nearly  always  muddy.  There  is  also  a 
considerable  invisible  load  that  all  streams  carry  in  solution. 

Corrosion 

This  is  the  solution  work  of  streams;  it  is  sometimes  called  chemical 
denudation.  Corrosion  is  closely  connected  with  weathering  and 
ground-water  work,  since  the  ground  water  in  its  slow  journey  leaches 
out  soluble  materials,  some  of  which  are  carried  to  streams.  Because 
of  this  it  is  clear  that  much  of  the  invisible  load  is  brought  to  streams  by 
ground  water  rather  than  by  the  rapidly  moving  run-off  water.  A 
stream  corrodes  its  channel  to  a  very  slight  extent  and  its  fine  load 
also  gives  up  some  soluble  materials,  but  these  are  minor  sources  of 
supply.  Obviously  the  amount  carried  in  solution  by  streams  is  closely 
related  to  the  rocks  of  the  stream  basin.  Streams  draining  a  lime- 
stone basin  will  ordinarily  carry  much  more  soluble  materials  than 
those  from  a  sandstone  region  and,  other  things  being  equal,  stream? 


CORRASION  135 

from  regions  covered  with  materials  ground  up  by  glaciers  will  con- 
tain considerable  soluble  material.  It  is  estimated  that  the  Mis- 
sissippi River  is  lowering  its  basin  one  foot  in  25,000  years  by  corrosion. 
Murray  has  estimated  that  nineteen  principal  rivers  of  the  world  carry 
the  following  amounts  in  solution : 

Constituents.  Tons  in  cubic  mile. 

Calcium  carbonate  (CaCO3) 326,710 

Magnesium  carbonate  (MgCO3) 112,870 

Calcium  phosphate  (Ca^P2O8) 2,913 

Calcium  sulphate  (CaSO4) 34,301 

Sodium  sulphate  (Na2SO4) 31,805 

Potassium  sulphate  (K2SO4) 20,358 

Sodium  nitrate  (NaNO3) 26,800 

Sodium  chloride  (NaCl) 16,657 

Lithium  chloride  (LiCl) '. 2,462 

Ammonium  chloride  (NH4C1) 1,030 

Silica  (SiO2) " 74,577 

Ferric  oxide  (Fe2O3) 13,006 

Alumina  (A12O3) 14,315 

Manganese  oxide  (Mn2O3) 5,703 

It  is  at  once  evident  that  lime  and  magnesium  carbonates  con- 
stitute the  bulk  of  the,  dissolved  materials  and  the  lime  phosphate  and 
sulphate  of  potash  show  the  continued  loss  of  valuable  plant  food.  In 
the  humid  regions  the  decaying  vegetation  supplies  carbonic  acid  and 
the  soluble  compounds  are,  therefore,  largely  carbonates,  but  in  arid 
regions,  where  tnis  agent  is  for  the  most  part  subordinate,  the  more 
common  compounds  are  sulphates  and  chlorides.  Thus  many  rivers  in 
the  western  Mississippi  Basin  which  flow  from  semi-arid  through  humid 
regions  show  an  increase  in  carbonates  in  solution  from  source  to  mouth 
and  an  increase  in  chlorides  and  sulphates  in  the  opposite  direction. 

Corrasion 

This  term  is  applied  to  the  wear  and  tear  of  the  stream  load  upon  its 
channel  and  of  the  particles  of  the  load  upon  themselves.  Clear 
water  in  itself  has  little  power  to  corrade  and  it  is  only  when  the  current 
is  supplied  with  rock  as  tools  that  corrasion  is  effective.  The  Niagara 
and  St.  Lawrence,  both  clear  rivers,  show  the  inefficiency  of  clear  water 
in  corrasion.  At  the  very  brink  of  the  American  Falls  where  the  cur- 
rent is  swift  there  are  delicate  thread-like  plants  (algae)  which  would 
shortly  be  scoured  away  were  there  any  considerable  quantities  of  sand 


136 


STREAMS  AND  THEIR  WORK:  ALLUVIAL  SOILS 


or  pebbles  carried;   the  upper  St.  Lawrence  has  not  yet  removed  the 
delicate  glacial  scratches  on  the  rock  of  its  channel. 

The  tools  by  which  a  stream  can  corrade  are  the  rocks,  pebbles, 
sand  and  silt  carried  by  the  stream  which,  striking  upon  the  bottom  and 
colliding  with  each  other,  grind  and  chip  off  fragments  of  the  load. 
The  efficiency  of  corrasion  varies  primarily  with  the  stream's  velocity, 
but  the  size,  shape  and  hardness  of  the  materials  carried  are  important 
factors.  Under  the  rolling,  grinding  and  colliding  of  the  particles  the 
stream  load  becomes  rounded,  a  characteristic  shape  of  stream  peb- 
bles. Aside  from  active  factors,  the  rate  of  corrasion  is  obviously  influ- 
enced by  the  resistance  of  the  underlying  rock.  The  Colorado  River 


FIG.  109. — The  figures  in  the  different  districts  show  the  estimated  number  of  years 
required  for  the  land  to  be  reduced  one  inch  by  erosion.  For  example,  the  Ga., 
S.  C.,  N.  C.,  Va.  area  will  be  reduced  one  inch  in  710  years.  (After  National 
Conservation  Commission.) 

furnishes  an  example  of  rapid  corrasion  owing  to  its  volume,  high  veloc- 
ity and  ample  supply  of  rock  tools  by  which,  in  a  short  time,  geologically 
speaking,  it  has  cut  the  famous  Colorado  Canyon.  A  corrading  river 
"  may  be  compared  to  a  sinuous,  flexible  and  endless  file,  ever  moving 
forward  in  one  direction  .  .  .  and  rasping  away  the  country  rock 
beneath  "  (Pirsson  and  Schuchert).  '  The  combined  work  of  corrosion 
and  corrasion  in  lowering  the  land  is  shown  in  Fig.  109. 


THE  DEVELOPMENT  OF  VALLEYS  AND  DIVIDES  137 


The  Development  of  Valleys  and  Divides 

It  has  been  noted  before  that  practically  all  the  run-off  from  rainfall 
is  accomplished  through  streams.  The  depression  in  which  a  stream 
flows  is  its  valley,  while  the  area  drained  by  a  stream  and  its  tributaries 
is  its  basin.  The  growth  of  a  valley  can  perhaps  be  best  understood 
by  taking  as  an  example  a  small  valley  or  gully.  A  valley  begins  where, 
because  of  natural  drainage  lines,  weak  materials  or  perhaps  an  espe- 
cially heavy  rainfall,  there  is  started  a  line  of  drainage.  The  running 
stream  soon  cuts  a  trench  with  steep  slopes  at  the  head  and  the  trench 
is  widened  by  the  inflow  of  water  at  the  sides.  The  valley  grows  at  its 
head  because  the  slopes  here  are  steeper  and  the  water  runs  faster  and, 
therefore,  corrades  rapidly.  Such  erosion  at  the  heads  of  valleys  is 
known  as  head  erosion.  In  time  tributaries  will  develop  and  neigh- 
boring streams  will  cut  valleys  so  that  there  will  be  an  advancing  zone 
of  head  erosion  in  many  valleys.  The  combined  head  erosion  in  the 
valleys  sometimes  produces  a  united  series  of  valley  head  slopes  so  as 
to  form  a  fairly  continuous  steep  slope  or  escarpment. 

Fig.  110  shows  such  an  escarpment  in  the  Texas  Panhandle,  which  is  of  consider- 
able extent.  The  eastward  flowing  streams  to  the  right  (east)  are  extending  their 
valleys  westward  by  head  erosion 
and  invading  the  plain  which  formerly 
extended  far  to  the  eastward.  Thus 
there  are  three  belts,  the  level,  un- 
eroded  plain  at  the  left,  the  middle 
hilly  zone  of  head  erosion  or  "  brakes  " 
with  an  escarpment  and  the  rolling 
eroded  prairies  to  the  east,  each  having 
different  soils  and  drainage.  The  es- 
carpment, so  to  speak,  is  invading  the 
plain  to  the  west  and  leaving  in  its 
wake  the  rolling  prairies.  FIG.  110.— Head  erosion  of  several  streams 

'  Another  extremely  interesting  ex-       producing  an  escarpment,  Texas.     (Data 
ample  of  head  erosion  on  a  large  scale       after  Gould,  U.  S.  Geological  Survey.) 
is  seen   in  the  western  High  Plains  of 
the  western  Mississippi  Basin,  Fig.  111. 

This  area,  the  soils  of  which  are  composed  of  loose  unconsolidated  materials,  shows 
two  distinctive  regions,  one  flat  and  uneroded  and  the  other,  eroded  and  rolling,  while 
between  these  areas  is  a  more  or  less  well-marked  escarpment.  It  will  be  seen  that  the 
smooth  area  roughly  coincides  with  the  region  With  dry  climate  and  the  rolling  area, 
with  the  region  with  humid  climate.  The  rolling  area  is  well  covered  with  a  tena- 
cious sod  which,  to  a  considerable  degree,  affords  protection  from  erosion,  "not 
because  it  resists  the  work  of  well-developed  drainage  but  because  it  prevents  the 
initiation  of  drainage"  (Johnson).  On  the  other  hand,  the  streams  flowing  east- 


138  STREAMS  AND  THEIR  WORK:  ALLUVIAL  SOILS 

ward  have  pushed  back  their  headwaters  into  the  dry  country  and  are  pushing  the 
escarpment  westward  where  the  scanty  bunch  grass  of  the  dry  country  affords  but 
little  protection  against  erosion. 

A  valley  seldom  is  cut  straight  unless  it  is  guided  by  some  fairly 
straight  and  weak  stratum  as  in  Fig.  58,  but,  otherwise,  the  valleys 
tend  to  form  a  network  of  tributaries  much  like  the  branches  of  a  tree. 

The  first  type  is  likely  to  be 

HIGH    PLAINS  ROLLING    PLAINS  r  i      •  •  i  j  i 

SUB-HUMID  ,          HUMID  found  in   regions   where    the 

rocks  are  folded  or  tilted.  The 
factor  of  weathering  in  the 
widening  of  valleys  is  often 

unappreciated.     A  stream  it- 
FIG.  111. — Diagram    to    illustrate    the    sub-          lf  ,     ,    r, ,,  ,, 

•j    TT-  u    ™  •       ±u     u      M   T»  ir         self  cuts  but  little  more  than 
humid    High    Plains,  the    humid    Rolling 

Plains  and  the  dividing  escarpment.  (Modi-      the    stream    width,    but,    as 
fied  from  Johnson.)  the    stream    cuts    its    valley, 

the    processes    of   weathering 

on  the  valley  sides  weaken  the  rocks  and  cause  them  to  fall  and  the 
rain  wash  carries  the  debris  into  the  stream.  As  a  consequence  prac- 
tically all  valleys  are  flaring  toward  the  top.  Even  the  Colorado 
Canyon  has  a  flaring  cross-section,  although  it  is  very  young  and  it  has 
been  cut  in  resistant  rocks  and  the  weathering  in  that  region  is  com- 
paratively slow. 

A  divide,  as  the  name  implies,  is  a  line  between  two  streams  where 
the  rainfall  separates,  a  portion  flowing  from  the  divide  into  each  stream. 
A  divide  may  be  sharp  or  indistinct  and  flat,  high  or  low.  Divides  are 
sharpened  by  the  widening  of  valleys  or  by  the  headward  extension  of 
streams.  As  valleys  are  widened  and  the  tributaries  push  their  heads 
away  from  the  main  streams,  the  divides  become  narrower.  Many  high 
and  rugged  divides  constitute  mountains,  a  good  example  of  which  is 
the  Blue  Ridge  Mountains  of  North  Carolina. 

Incised  Meanders 

Few  streams  are  straight  but  most  flow  in  more  or  less  well-marked 
curves  or  meanders  which  are  cut  into  the  underlying  rock  and  become 
winding  trenches  or  incised  or  entrenched  meanders  as  they  are  variously 
called.  Incised  meanders  are  important  agents  in  widening  valleys 
and  their  work  will,  therefore,  be  considered  in  some  detail. 

When  a  stream  flows  in  a  meander  it  does  not  cut  equally  on  both  sides, 
but  it  cuts  more  on  the  outside  of  the  meander,  for  here  is  the  fastest 


INCISED   MEANDERS 


139 


current.  This  process,  continued  for  a  long  time,  will  result  in  a  stream's 
widening  its  valley  and  eventually  wearing  away  the  protecting  spurs 
as  shown  in  Fig.  112.  Finally,  the  valley  is  widened  with  perhaps  here 
and  there  a  remnant  of  a  spur  that  once  projected  far  into  the  valley. 
The  flat  bottom  of  the  valley  is  an  alluvial  plain. 

Looking  again  at  the  diagram, 
Fig.  112,  in  A  the  stream  is 
actively  cutting  and,  therefore, 
is  termed  a  degrading  stream. 
In  B,  C,  and  D  the  stream  is 
neither  actively  cutting  or  de- 
positing and  is  just  able  to  carry 
its  load  so  it  is  termed  a  graded 
stream.  When  a  stream  is  ac- 
tively building  it  is  termed  an 
aggrading  stream. 

While  the  soils  that  are 
typically  associated  with  incised 
meanders  are  not  extensive,  yet 
they  are  important  locally  and 
merit  some  consideration.  An 
example  of  an  incised  meander 
of  the  Ohio  River  and  its  asso- 
ciated soils  is  shown  in  Fig.  113. 
(A)  represents  an  early  stage  of 
the  meander  where  the  river  is 
cutting  most  on  the  left-hand 
bluffs.  Now  turning  to  Fig.  114, 
which  represents  a  small  portion 
of  (A)  in  Fig.  113  we  note  that 
th'e  stream  cuts  both  vertically  and  laterally  and,  as  a  result,  its  down- 
ward cutting  will  be  a  combination  of  these  two  movements  as  shown 
by  the  arrows.  Two  characteristic  slopes  will  result  from  this  double 
cutting,  one  a  steep  slope  at  the  left  and  the  other  a  gentle  slope  at 
the  right.  The  stream  swinging  outward  keeps  the  left  slope  steep, 
while  on  the  right  slope  the  stream  "  slips  off,"  so  to  speak.  In 
other  words,  the  stream  has  traveled  down  or  "  slipped  off  "  the  gentle 
slope  and  some  successive  positions  of  the  stream  are  shown  at  S,  S,  S. 

Now  turning  to  (B)  of  Fig.  113,  we  see  that  the  river  has  slipped 
off  its  long  slope  and  is  undercutting  at  the  left.     Another  feature  here 


FIG.  112. — Stages  in  the  formation  of  incised 
meanders.     (Davis.) 


140 


STREAMS  AND  THEIR  WORK:   ALLUVIAL  SOILS 


is  the  level  alluvial  plain  which  is  appearing  at  the  foot  of  the  slip-off 
slopes.  The  river  has  become  graded,  that  is,  it  has  ceased  cutting 
vertically  and  is  widening  its  valley  only  by  lateral  cutting.  In  (C)  a 


ALLUVIAl  ..:TV=2--' 
SOILS        


FIG.  113. — Diagrams  to  illustrate  the  formation  of  an  incised  meander  and  its  associ- 
ated soils. 

later  stage  is  represented  which  has  been  sketched  from  the  combined 
characteristics  of  several  meanders.  The  river  is  widening  its  valley 
on  the  left  and  eroding  the  projecting  spur  on  its  right  side.  It  has 

moved  laterally  so  far  that  a  wide  ex- 
panse of  level  alluvial  land  has  been 
developed.  Fig.  (D)  shows  the  char- 
acteristic soils  of  an  incised  meander, 
Where  the  river  is  still  cutting  later- 
ally the  slopes  are  kept  steep  with  the 
result  that  head  erosion  is  so  active 
that  the  finer  soil  particles  are  swept 
away  and  the  soils  are,  therefore,  stony. 
The  strips  of  alluvial  lands  follow  the  river  as  it  advances  by  lateral 
cutting.  In  some  valleys  one  can  look  up  stream  and  see  a  succession 
of  smooth  slip-off  slopes  often  in  cultivation  while,  looking  in  the 
opposite  direction  the  rugged  cliffs,  due  to  undercutting,  are  in  view. 


FIG.  114. — To  illustrate  the  "slip- 
ping off"  of  an  incised  meander. 


Soil  Erosion 

The  principles  concerning  the  development  of  shallow  valleys  and 
their  associated  divides  have  important  applications  in  soil  erosion,  a 
very  important  agricultural  problem  in  many  places.  All  soils,  except 
those  on  flat  surfaces,  are  subject  to  some  erosion,  but,  under  favorable 
circumstances,  the  ratio  of  soil  washing  above  is  about  balanced  by  the 
formation  of  soil  below.  Sheet  washing  occurs  where  practically  no 
gullies  are  formed  and  the  surface  soil  sometimes  to  a  depth  of  an  inch 
or  more  is  carried  away  by  a  sheet  of  water  during  a  heavy  rain.  The 


SOIL  EROSION 


141 


more  noticeable  type  is  "gully  washing/'  where  gullies  form  and 
sometimes  grow  for  several  yards  during  one  rainfall.  Both 
processes  usually  go  on  together,  but  sheet  washing  is  usually 
not  noticed  because  it  does  not  notably  alter  the  surface.  "  Gully 
washing/'  on  the  other  hand,  tends  to  render  the  land  too  rough  for 
cultivation. 

Factors. — The  factors  contributing  to  soil  erosion  are  many.     (1) 
Perhaps  the  most  striking  cause,  because  the  results  are  so  soon  evident, 
is  the  removal  of  forests,  Fig.  115.     So  long  as  the  interlacing  tree  roots 
hold  the  soil  and  the  accumulated  muck  and  leaves  absorb  the  rainfall 
and  retard  the  run-off,  there  is 
little  soil  erosion  even  in  forests 
where  the  slopes  are  steep  and 
other  conditions   are   favorable. 
The  cutting  of  forests  on  moun- 
tain slopes  has  ruined  thousands 
of  acres  for  all  practical  purposes. 
This  is  unfortunately  well  shown 
in  many  parts  of  the  Appalachi- 
ans, but  the  classical  example  is 
seen  in  the  Karst  region  of  south- 
western Austria.  Here,  in  Roman 
times,  was  a  heavily  forested  hilly 
region  but,  with  the  reckless  de-   FIG.  115. — Destruction   of   the   woodland 
forestation,  the  hills  were  swept       without  adequate  reforestation  has  caused 
bare  of  soil  and  to-day  most  of  the      gullying'     :U"  S"  Forest  Service-} 
Karst  is  practically  a  desert.     (2) 

Somewhat  the  same  result  follows  the  loss  of  a  protective  sod  cover. 
In  many  regions  of  somewhat  scanty  rainfall  and  steep  slopes,  unwise 
plowing  and  too  close  grazing  have  destroyed  the  sod  and  made  con- 
ditions favorable  for  soil  erosion.  (3)  An  obvious  factor  favoring  soil 
erosion  is  steep  slopes  which  make  for  rapid  run-off  and  therefore 
greater  corrasion.  (4)  Soil  texture  is  important.  A  clay  or  silt  soil 
absorbs  water  slowly  so  that  the  run-off  is  high  while  sandy  soils 
absorb  water  rapidly  and  only  heavy,  prolonged  rains  will  soak  the 
soil  so  that  it  is  notably  subject  to  erosion.  Furthermore,  fine-grained 
soils  are  more  easily  carried  away.  (5)  Unwise  plowing  and  culti- 
vating up  and  down  hill  are  responsible  for  much  sheet  erosion  and  gully- 
ing. Shallow  plowing  by  failing  to  provide  loosened  soil  to  soak  up 
rainfall  is  also  a  fruitful  cause  of  soil  erosion.  (6)  A  cultivation  which 


142 


STREAMS  AND  THEIR  WORK:  ALLUVIAL  SOILS 


depletes  soil  of  its  humus  renders  erosion  more  probable,  since  humus 
absorbs  water  and  thereby  decreases  the  run-off. 

Remedies  for  soil  erosion  in  part  suggest  themselves  and  grow  out 
of  the  principles  of  valley  making.  Rainfall  cannot  be  controlled 

nor  can  the  soils  be  materially 
changed,  so  the  best  remedy 
4  is  prevention — not  to  allow 
the  head  erosion  to  start.  A 
sod  is  the  best  protection  and 
sod  on  hill  pastures  should 
not  be  killed  by  too  close 
grazing,  but  cover  crops  will 
protect  the  soil  fairly  well 
where  there  is  no  sod.  Deep 
plowing  allows  the  soil  to  ab- 
sorb more  water  and  so  tends 
to  reduce  the  run-off.  Plenty 
of  organic  matter  in  the  soil 
will  absorb  water,  reduce  the 
run-off  and  tend  to  bind  the 
soil;  this  fact  explains  why 
old  soils  from  which  organic 
matter  has  been  removed  will 

wash  worse  than  fresh  soils.  Contour  plowing  will  allow  cultivation 
because  the  contour  furrows  tend  to  retard  the  wash  and  prevent  the 
formation  of  gullies. 

When  small  valleys  have  started  it  is  very  important  to  check  them 
early  for,  once  started,  they  become  deeper  in  the  lower  portions,  thus 
increasing  the  slopes  and  making  cumulative  conditions  favorable  for 
growth.  With  the  growth  of  such  valleys,  not  only  is  the  surface 
rendered  too  rough  for  cultivation,  but  soil  and  mantle  rock  are  swept 
onto  productive  soils  and  they  are  made  useless.  Moreover,  the 
deepening  of  gullies  lowers  the  water  table  over  adjacent  areas,  thus 
making  the  crops  sensitive  to  droughts.  By  depositing  straw,  brush  or 
other  litter  at  the  head  of  gullies,  the  head  slopes  and,  therefore,  the  head 
erosion  are  decreased  and,- moreover,  the  rains  sink  into  the  loose  mate- 
rials thereby  decreasing  the  run-off.  If  land  values  are  high  enough 
to  warrant  the  expense,  dams  are  sometimes  run  across  valleys, 
sediment  collects  behind  them  and  the  gullies  in  time  become 
nearly  filled.  Terraces,  Fig.  117,  have  long  been  used  in  with 


FIG.  116. — Checking  of  soil  erosion  by  brush 
dams.     (U.  S.  Geological  Survey.) 


SOIL  EROSION  143 

valuable  lands;  they  reduce  the  slope  and  thereby  lessen  the  run-off 
and  corrasion. 

Bad  land  topography  is  due  to  excessive  gullying,  Fig.  118.  It  occurs 
most  typically  in  portions  of  western  Nebraska,  Wyoming  and  the 
Dakotas  where  incoherent  materials,  scanty  soil  cover  and  torrential 
rainfall  are  favoring  conditions.  Such  areas  are  chiefly  valuable  for 
grazing,  but  excessive  grazing  by  reducing  the  cover  will  aid  in  the 
development  of  this  topography.  Extensive  bad  land  areas  have  been 


FIG.  117. — Terraces  in  Central  China.  The  terraces  both  reduce  erosion  and  aid 
in  making  the  steep  hill  sides  available  for  cropping.  (Willis,  Smithsonian 
Institution.) 

produced  in  the  loose  materials  of  the  Lafayette  Formation  in  some  of 
the  Southern  states  by  neglect  of  the  soil  cover. 

STREAM  TRANSPORTATION 

Factors. — The  familiar  fact  that  streams  carry  materials,  of  load 
as  it  is  called,  has  been  noted  under  the  topic  of  corrasion.  It  is  a 
matter  of  common  observation  that  streams  in  high  water  and  with 
consequent  greater  velocity  can  carry  larger  and  heavier  masses  than  the 
same  streams  at  low  water  with  slower  current.  The  carrying  power  of 
streams  increases  very  rapidly  with  the  velocity  for,  if  a  stream's 
velocity  is  doubled,  the  carrying  power  is  increased  64  times  or,  in 


144  TREAMS  AND  THEIR  WORK:    ALLUVIAL  SOILS 

other  words,  the  transporting  power  of  a  current  varies  as  the  sixth 
power  of  the  velocity.  With  this  in  mind  one  can  understand  how  a 
small  stream  in  flood  can  transport  even  large  boulders.  The  efficiency 
of  Velocity  in  stream  transportation  is  illustrated  by  the  jetties  which 
were  built  at  one  of  the  mouths  of  the  Mississippi  River.  In  order  to 
overcome  the  tendency  of  the  river  to  deposit  its  sediment  and  obstruct 
the  channel,  these  embankments  or  jetties  were  built,  one  on  either  side, 
to  confine  the  river,  make  it  flow  faster  and  thereby  keep  the  channel 


FIG.  118.— "Bad  Land"  topography,  Neb.     (U.  S.  Geological  Survey;  from  Forest 
Physiography  by  Isaiah  Bowman,  Wiley  &  Sons,  Inc.) 

clear.     Most  streams  are  swiftest  in  their  upper  courses  and  the  coarsest 
load  is  usually  found  here. 

Other  factors  in  stream  transportation  are  the  size,  weight  and  shape 
of  the  materials  making  the  load.  For  example,  a  stream  might  not 
be  able  to  move  a  stone  weighing  a  pound,  but  if  the  stone  is  broken 
into  small  fragments  the  stream  might  easily  carry  them  away,  because 
the  total  surface  against  which  the  currents  could  strike  would  be 
enormously  increased.  In  other  words,  the  larger  the  surface  of  a 


STREAM  TRANSPORTATION  145 

particle,  the  more  easily  it  may  be  moved,  other  things  being  equal. 
Thus  it  is  clear  why  the  bulk  of  stream  loads  is  composed  of  sand,  silt 
and  clay,  which,  though  they  may  be  very  small,  present  large  sur- 
faces in  proportion  to  their  weight.  The  following  table  gives  an  idea 
of  the  transporting  power  of  bottom  currents  in  rivers.1 

Velocity  of  current.  Size  of  materials  carried. 

3  inches  per  second,  about    £  mile   per  hour Fine  clay  and  silt. 

6  inches  per  second,  about    ^  mile   per  hour Fine  sand 

2  feet  per  second,  about      1£  miles  per  hour Pebbles  1  inch  in  diameter 

4  feet  per  second,  about      2|  miles  per  hour Pebbles  4  inches  in  diameter 

The  Stream  Load  in  Transit. — The  stream  load  is  moved  in  two  ways. 
The  heavier,  coarser  sand  and  gravel  are  rolled  and  pushed  along  the 
bottom,  while  much  of  the  fine  load,  like  the  fine  sand,  silt  and  clay 
is  carried  in  suspension.  The  first  method  is  characteristic  of  swift 
streams  like  those  in  mountains;  slower  streams  carry  much  of  their 
loads  in  suspension  somewhat  as  dust  is  carried  in  the  air.  The  ability 
of  running  water  to  carry  materials  heavier  than  itself  is  due  mainly  to 
three  factors:  (1)  The  impact  of  the  moving  water  against  a  particle 
hurls  it  on  a  journey  somewhat  as  a  ball  is  projected  from  a  bat.  (2) 
When  rocks  are  immersed  in  water,  they  suffer  an  apparent  loss  of 
about  one-third  of  their  weight,  a  principle  which  aids  a  stream  in 
carrying  its  fine  load  in  suspension.  (3)  Very  small  particles,  once  they 
are  suspended  in  water,  sink  very  slowly  owing  to  friction  and,  therefore, 
remain  suspended  even  without  the  aid  of  currents.  A  glass  of  river 
water  will  often  not  become  clear  for  weeks.  From  this  it  should  not  be 
thought  that  particles  remain  indefinitely  suspended;  rather  they  jour- 
ney by  long  or  short  leaps  with  rests  between  the  journeys.  Much  of 
the  load  is  moved  in  floods  and  rests  at  ordinary  stages,  and  the  load  as  a 
whole  travels  much  more  slowly  than  the  water. 

Furthermore  there  are  numerous  upward  currents,  the  larger  of 
which  show  as  "  boilings,"  which  carry  materials  upward  in  the  stream. 
Then  it  must  be  understood  that  a  stream  is  not  a  single  current,  but  is 
composed  of  many  currents,  slanting,  vertical,  even  backwards,  and 
these  keep  the  water  agitated  so  that  fine  materials  do  not  readily  sink. 

The  load  of  rivers  is  largely  derived  from  their  smaller  tributaries. 

although  small  amounts  may  be  picked  up  from  the  channels,  and  the 

amount  of  the  load  is  obviously  dependent  on  the  materials  of  the 

channel  and  basin — whether  they  are  of  resistant  rock  or  of  unconsol- 

1I.  C.  Russell,  Rivers  of  North  America,  page  18. 


146 


STREAMS  AN0  THEIR  WORK:  ALLUVIAL  SOILS 


idated  materials.  Sheet  wash,  soil  creep  and  slumping  down  the  sides 
of  small  rill  valleys  furnish  load  to  the  small  streams  which  they  transfer 
to  the  larger  streams.  Furthermore  it  must  be  remembered  that, 
because  a  stream  is  competent  to  carry  a  given  load,  it  may  not  actually 
carry  that  load.  The  stream  can  carry  only  the  sediment  that  is  brought 
to  it.  Mountain  streams  of  high  velocity  are  often  clear  because  the 
rocky  slopes  do  not  furnish  sediment  that  the  streams  could  carry  and 
even  a  muddy  river  may  not  be  loaded  to  its  capacity  because  the  load 
fe  so  fine  that  the  stream  can  easily  carry  it.  The  following  estimates 
are  interesting  in  comparing  different  rivers  :* 

DRAINAGE  AND  SEDIMENT  OF  LARGE  RIVERS 


Average 

SEDIMENT. 

• 

Rivers. 

Drainage 
area  in 
square 
miles. 

annual 
discharge  of 
water 
in  cu.  ft. 
per  second. 

Total  annual 
tons. 

Ratio  of  sed- 
iment to 
water  by 
weight. 

Height  in 
feet  of 
column, 
one  square 
mile  base. 

Mississippi 

1,244,000 

610,000 

406,250,000 

to  1,500 

241  4 

Potomac  

11,043 

20,160 

5,557,250 

to  3,575 

4.0 

Rio  Grande  

30,000 

1,700 

3,830,000 

to     291 

2.8 

Po. 

27,100 

62,200 

67,000,000 

to     900 

59  0 

Danube  

320,300 

315,200 

108,000,000 

to  2,880 

93.2 

Nile    .    . 

1,100,000 

113,000 

54,000,000 

to  2,050 

38.8 

Irawaddy. 

125,000 

475,000 

291,430,000 

to  1,610 

209  0 

While  the  Mississippi  carries  more  sediment  than  any  of  the  others, 
yet  its  load  (ratio  of  sediment  to  water)  is  less  than  several  others,  the 
Potomac  for  instance.  The  Irawaddy  River  with  a  much  smaller  basin 
carries  a  load  that  will  compare  with  that  of  the  Mississippi  doubtless 
because  of  the  heavy  rainfall  in  the  basin  of  the  former  river. 

Abrasion  of  the  Load  in  Transit. — In  large  rivers  the  finer  load  is 
found  in  the  lower  reaches  and,  in  general,  most  streams  show  an 
increase  in  the  size  of  particles  carried  as  one  goes  up  stream.  In  the 
first  place,  the  currents  in  the  lower  parts  of  rivers  are  usually  weaker 
and  less  variable  and  can  transport  only  fine  materials  like  fine  sand, 
silt  and  clay.  Then,  in  a  long  river,  the  particles  of  the  load  have  been 
exposed  to  the  corrasion  that  accompanies  stream  transportation  and 
thus  have  been  worn  to  smaller  dimensions  by  their  journey.  This 
.'  ;  *  Babb,  quoted  by  Russell,  page  74. 


STEAM   TRANSPORTATION  147 

is  well  shown  in  the  following  table  made  by  Hochenburger  as  the 
result  of  studies  made  of  the  Mar  River  in  Europe:1 

Distance  carried.  Average  size  of  fragments. 

At  Gratz 224  cu.  cm. 

5 . 68  miles  below 184  cu.  cm. 

14 . 76  miles  below 132  cu.  cm. 

24 . 42  miles  below 117  cu.  cm. 

31.81  miles  below 31  cu.  cm..    . 

40 . 33  miles  below 60  cu.  cm. 

57  36  miles  below 33  cu.  cm. 

68 . 16  miles  below 21  cu.  cm. 

In  a  journey  of  about  seventy  miles  the  fragments  have  been  abraded 
to  about  one-tenth  their  former  size.  This,  of  course,  does  not  apply 
to  all  streams,  but  the  table  illustrates  the  comminution  of  load  with  its 
journey  down  stream.  The  same  author  gives  the  following  table  to 
show  the  resistance  to  corrasion  of  different  rocks  in  the  same  river; 
the  figures  give  the  distance  in  miles  each  rock  travels  before  becoming 
thoroughly  broken  into  very  small  fragments: 

Rhaetic  sandstone 8 . 52  miles 

Clay  state * 23 . 85  miles 

Orthoceras  limestone 36 . 35  miles 

Grabular  limestone 48 . 28  miles 

Granite 157 .90  miles 

Another  point  of  great  agricultural  interest  is  the  fact  that,  as  the 
particles  in  the  stream's  load  become  smaller,  they  suffer  less  abrasion 
in  transit  mainly  because  the  blows  that  they  strike  against  each  other 
and  against  the  stream  channel  become  lighter  as  the  weights  of  the 
particles  decrease.  As  a  result  the  weaker  but  important  minerals 
such  as  feldspar  and  apatite  are  carried  for  long  distances  and  deposited 
as  silt  and  clay  in  particles  so  small  that  they  the  more  readily  give 
up  their  plant  food  to  roots;  even  a  mineral  so  apparently  frail  as  mica 
is  carried  long  distances  and  is  a  not  uncommon  soil  mineral  in  alluvial 
soils.  This  principle  is  illustrated  in  Fig.  119,  which  shows  that  most 
sand  grains,  easily  visible  to  the  unaided  eye,  are  rounded  by  their 
journey,  but  that  most  of  the  small  microscopic  grains  are  angular. 

STREAM  DEPOSITS.     ALLUVIATION 

Stream  deposition  is  the  converse  of  stream  transportation  and 
therefore  anything  that  lessens  a  stream's  carrying  power  tends  to  pro- 
1  Quoted  by  Grabeau,  Principles  of  Stratigraphy. 


148 


STREAMS  AND  THEIR  WORK:   ALLUVIAL  SOILS 


mote  deposition.  The  process  of  stream  deposition  is  sometimes 
termed  alluviation,  and  the  results,  alluvial  deposits.  This  process  pro- 
duces alluvial  soils  which  are  important  in  extent,  productiveness  and 
value. 


Factors 

Diminished  Velocity.  —  Since  the  carrying  power  of  streams  increases 
very  rapidly  with  increasing  velocity,  it  follows  that  a  reduction  of 


FIG.  119. — Fine  river  sediments.  On  the  left  are  grains  of  rounded  sand,  magnified; 
the  grains  are  about  -£g  of  an  inch  in  diameter.  On  the  right  are  minute  grains 
of  silt,  much  magnified;  the  grains  are  about  5^  °f  an  mcn  in  diameter. 
The  smaller  grains  are  much  less  abraded  than  the  larger  grains. 


carrying  power  will  accompany  lessening  velocity.  For  example,  a 
stream  that  is  just  able  to  carry  coarse  sand  will  drop  that  portion  of 
its  load  when  the  current  is  slightly  lessened.  It  will  be  remembered 
that  a  stream  is  a  complex  of  currents,  fast  and  slow,  so  that  one  place 
it  may  be  carrying,  and  elsewhere,  depositing  sediment.  Not  only  so, 
but  even  the  same  current  may  simultaneously  deposit  its  coarse  load 
and  carry  its  fine  load.  Moreover,  when  a  stream  drops  its  heavy 
load,  some  of  its  energy  is  released  and  it  may  be  able  to  take  up  and 
carry  more  fine  materials  if  they  are  available,  thereby  effecting  an 
exchange  of  load. 

Diminished  Volume. — We  have  seen  that  volume  directly  affects 
the  carrying  power  of  streams.  Most  streams  have  one  or  more  high- 
water  stages  during  which  more  and  coarser  sediments  are  carried 
and  deposited,  thus  producing  a  succession  of  different  layers  or  strata 


FACTORS  149 

so  that  stream  deposits  are  commonly  stratified.  The  Nile  is  a  famous 
example  of  a  depositing  river.  During  the  rainy  season  it  acquires  at 
its  headwaters  a  large  volume  by  which  it  is  able  to  carry  its  fine  load 
for  hundreds  of  miles  and  finally  deposits  the  sediment  along  its  lower 
course.  Some  rivers  rise  in  a  humid  region  and  flow  for  a  part  of  their 
courses  through  a  dry  region,  and  here  they  lose  much  of  their  volume 
through  seepage  and  evaporation.  As  a  result  of  the  diminished  vol- 
ume the  stream  drops  much  of  its  load  in  its  channel  and  flows  in  small 
streamlets  through  masses  of  its  deposits.  Such  streams  are  termed 
braided  streams  because  of  their  characteristic  appearance.  The  volume 
of  streams  is  greater  in  their  lower  courses  but  their  velocity  is  usually 
less  because  the  slope  is  low.  Other  things  being  equal,  a  rising  stream 
will  deposit  relatively  coarse  sediments  which  will  be  followed  by  finer 
sediments  as  the  stream  subsides. 

The  load  itself  is  an  important  factor  in  stream  deposition.  Obvi- 
ously a  lightly  loaded  stream  cannot  deposit  thick  sediment.  The 
fineness  and  weight  of  the  load  will  determine  how  far  the  load  can  be 
carried.  Streams  often  become  overloaded  locally,  an  instance  of  which 
is  seen  where  tributaries  bring  more  sediment  than  the  main  stream 
can  transport.  For  example,  some  of  the  rapids  in  the  Colorado  River 
are  explained  by  the  fact  that  a  tributary  brings  a  load  of  boulders 
which  the  main  stream  cannot  readily  carry.  From  these  varying 
factors,  sometimes  co-operating  and  sometimes  antagonistic  but  always 
varying,  we  have  the  marked  variations  in  alluvial  deposits.  Some 
alluvial  soils  are  extremely  variable,  while  other  single  types  cover 
large  areas,  but  have  frequent  minor  variations,  and  the  same  is  true  of 
all  sedimentary  deposits  and  rocks. 


CHAPTER  VIII 
CLASSES  OF  ALLUVIAL  DEPOSITS 

For  convenience  it  is  customary  to  classify  stream  deposits  as 
follows:  stream  channel  deposits;  flood  plains  or  alluvial  plains',  terraces; 
deltas;  alluvial  fans. 

Alluvial  Deposits  in  Channels 

Even  rocky  channels  usually  contain  some  sediment.  Many  streams 
flowing  in  narrow  valleys  have  thrown  down  some  of  their  loads  and 
partly  filled  their  valleys,  and  this  is  especially  true  of  streams  flowing 
from  regions  covered  with  glaciers  which  give  the  streams  so  heavy  a 
load  that  much  of  it  is  deposited  in  the  valleys  which  are  often  deeply 
filled.  Channel  deposits  are  often  very  heterogeneous,  varying  both 

vertically  and  laterally,  as  shown 
in  Fig.  120.  Such  variations  are 
due  to  the  highly  complex  and 
varying  stream  currents  each 
capable  of  carrying  different  loads. 
Bars  and  islands  are  often 
built  in  heavily  loaded  streams. 
The  lodging  of  a  tree,  for  example, 
will  often  create  an  obstruction 
back  of  which  deposition  may 
occur  and  the  materials  already 
deposited  create  a  still  greater 
obstacle  and  further  the  process. 


FIG.  120. — Section  of  Missouri  River  de- 
posits showing  varying  character  of 
the  sediments.  (After  Todd,  U.S. 
Geological  Survey.) 


Such  a  deposit  may  remain  under 
water  as  a  bar  or  it  may  be  built 

to  the  flood  surface  and  remain  as  an  island  when  the  water  is  lowered. 
The  surface  is  soon  covered  with  vegetation,  which  itself  retards  the 
velocity  of  the  water  and  promotes  the  growth  of  the  island.  Such 
islands  are  usually  more  or  less  temporary,  many  being  soon  eroded  and 
some  being  transferred  down  stream^ 

150 


FLOOD  PLAINS 


151 


1820 


1858 


An  illustration  of  the  latter  case  is  seen  in  Fig.  121.  Holmes  Island  in  the  lower 
Missouri  River  moved  several  miles  down  stream  in  fifty-five  years  (1820-1875). 
The  river  impinging  against  the  upper  part  of  the  island  wore  it  away  while  in  the 
more  quiet  waters  below  the  island,  sediment  was  deposited.  By  1875  the  river 
had  shifted  its  course  entirely  away  from  the  island. 

Flood  Plains 

Flood  plains,  as  the  name  implies,  are  plains  that  are  built  by  streams 
mainly  during  high  water.  They  are  known  variously  as  "  alluvial 

plains/'  "  river  bottoms,"  "  first 
bottoms,"  etc.  Flood-plain  soils  are 
proverbially  fertile  and  their  wide 
extent  of  available  land  makes  them 
the  most  important  alluvial  deposit 
from  an  agricultural  point  of  view. 
Origin. — When  a  stream  is  locat- 
ed in  a  level-bottomed  valley  which 
is  at  times  overflowed,  the  conditions 
are  favorable  for  building  a  flood 
plain.  A  stream  ,  at  high  water 
spreads  over  a  valley  and  deposits 
sediment,  usually  but  a  very  small 
fraction  of  an  inch  in  thickness,  but 
these  deposits  continued  for  a  long 
time  result  in  the  building  of  an 
alluvial  plain.  As  a  stream  over- 
flows, its  waters  are  spread  over 

larger  areas  and  the  friction  is  increased  with  a  resulting  lessening  of 
velocity  It  has  been  noted  that  streams  are  usually  slower  near  the 
margin  and  the  spreading  of  flood  waters  over  a  plain  virtually  extends 
the  slowe.  margin  waters.  Moreover,  a  stream  in  flood  usually  is  more 
heavily  laden  with  sediment,  and  this  factor  in  connection  with  the 
decreased  velocity  contribute?  to  the  building  ability  of  aggrading 
streams.  Finally  there  is  nearly  always  more  or  less  vegetation  on  a 
flood  plain  and  this  favors  deposition  because  it  lessens  the  velocity. 

Natural  Levee  (Front  Lands)  and  Back  Lands. — The  velocity  of  an 
overflowing  stream  is  not  uniformly  checked  over  the  entire  over- 
flowed area  and  the  first  and  most  marked  checking  occurs  near  the 
stream.  Here,  as  the  stream. loses  its  velocity  and  its  carrying  power, 
the  coarser  and  heavier  sediment  is  dropped  and,  in  addition,  some  of 


1875 


FIG.  121.— Diagram  to  show  the  down- 
stream movement  of  an  alluvial 
island  (dotted),  Mo. 


152 


CLASSES  OF  ALLUVIAL  DEPOSITS 


the  fine  sediment  is  also  deposited.  The  waters  farther  from  the  stream 
have  less  velocity  (1)  because  they  are  spreading  over  larger  areas  and 
thus  meeting  with  greater  resistance  and,  moreover,  (2)  the  slopes  at  a 
distance  from  the  stream  are  more  gentle  and  thus  cause  slower  cur- 
rents; these  slower  currents  carry  and  deposit  only  finer  materials  and 

also  relatively  small  amounts 
of  sediment.  (3)  Furthermore, 
there  is  often  back  water  which 
extends  back  from  overflowed 
areas  further  down  stream  and 
checks  the  current.  Indeed, 
at  the  beginning  of  floods  the 
back  water  may,  for  a  time, 
flow  in  a  -direction  opposite  to 
FIG.  122.—  Map  of  a  portion  of  the  Mississippi  that  of  the  main  stream. 


CLAY    JI  -^  MARSH  JL_- 


10ft.._. 
5  ..  — 


flood  plain  (above).  Below  is  a 
taken  across  the  center  of  the  map. 
from  U.  S.  Bureau  of  Soils.) 


profile          Since   at   high    water    the 
(Data  greatest  deposition  occurs  near 
the  ^^  {i  naturally  follows 

that  the  land  here  is  higher. 

The  higher  land  near  the  stream  is  called  the  natural  levee  or  often  locally 
front  land,  while  the  lower  land  farther  back  is  usually  termed  the  back 
land,  Fig.  122.  The  natural  levees  are  low  ridges  seldom  over  a  few  feet 


FIG.  123. — The  level  lower  Mississippi  flood  plain  looking  toward  the  river.    Win- 
drowed  sugar  cane  in  the  foreground.,  La. 

high  above  the  back  land  toward  which  they  descend  with  a  slope  so 
gradual  that  the  region  appears  flat,  Fig.  123.  During  high  water  the 
natural  levees  often  stand  out  as  long  low  islands  with  the  river  on  one 


FLOOD  PLAINS  153 

side  and,  on  the  other  side,  the  flood  water  covering  the  back  lands. 
Usually  the  natural  levee  is  so  high  that  tributaries  cannot  enter  directly, 
but  flow  for  considerable  distances  parallel  to  the  main  stream.  The 
Yazoo  River,  for  example,  flows  about  two  hundred  miles  along  the 
Mississippi  before  it  can  enter,  Fig.  136. 

Soils  of  Flood  Plains. — Not  only  is  the  land  usually  higher  near  the 
river,  but  there  is  typically  a  marked  difference  in  the  soils  of  the  front 
and  back  lands,  differences  in  composition,  texture  and  drainage.  The 
soils  on  the  front  lands  are  of  relatively  coarse  texture.  They  are 
usually  termed  "  sandy  "  although  they  maybe  silts;  however,  they 
nearly  always  contain  more  sand  than  the  back  land  soils,  hence  the 


FIG.  124. — The  "American  Bottoms."     Part  of  the  Mississippi  flood  plain,  Illinois. 

(U.  S.  Bureau  of  Soils.) 

common  name.  The  coarse-textured  soils  of  the  front  lands  usually 
grade  imperceptibly  into  the  finer  soils  of  the  back  lands  where  the 
prevalent  types  are  silts  and  clays.  In  this  connection,  it  is  perhaps 
worth  while  to  think  of  the  ideal  case  of  a  river  carrying  a  load  of  sand, 
silt  and  clay  and  overflowing  the  flood  plain  with  a  velocity  that  uni- 
formly decreases  from  the  stream  to  the  limits  of  the  back  lands.  Under 
such  conditions  there  would  be  successive  belts  from  the  stream  back  of 
sands,  sandy  loams,  loams,  silt  loams,  silts,  silty  clay  loams,  clay  loams 
and  clays.  While  such  an  ideal  arrangement  is  never  found,  yet  not 
infrequently  four  or  five  belts  of  these  soil  types  are  found  on  the  broad 
flood  plains  of  the  lower  Mississippi  and  other  large  rivers.  The 
changes  in  the  overflowing  currents  produce  many  variations  in  the 


154 


CLASSES  OF  ALLUVIAL  DEPOSITS 


ideal  arrangement.  An  actual  arrangement  of  soils  on  the  lower 
Mississippi  is  shown  in  Fig.  125,  which  shows  about  16  miles  of  flood 
plain.  The  Yazoo  fine  sandy  loam  (YS)  and  the  Yazoo  loam  (YL) 

are  front  land  soils  with  con- 
siderable sand.  The  Wabash 
clay  (WC)  and  the  Sharkey 
clay  (SC)  are  back-land  soils, 


FIG.  125. — Map  of  soil  types  on  a  part  of  the 
Mississippi  flood  plain.  (U.  S.  Bureau  of 
soils.) 


the  former  having  been  de- 
posited by  somewhat  swifter 
currents  than  the  latter.  The 
Wabash  clay  is  seen  fringing 

a  small  stream  in  a  wide  expanse  of  Sharkey  clay.  The  following  table 
gives  the  mechanical  analyses  of  these  different  types  and,  while  not 
typical  of  all  sections,  it  shows  the  change  from  relatively  coarse 
particles  to  finer  particles  from  the  river  to  the  back  lands:1 


,  ,  . 

Coarse 
sand. 

Medium 
sand. 

Fine 
Sand. 

Very  fine 
sand. 

Silt. 

Clay. 

Yazoo  fine  sandy  loam  (YS).  . 
Yazoo  loam  (YL).  .  

0.6 
0.4 

0.4 
0.2 

4.0 
1.3 

35.4 
3.0 

52.6 
72.4 

7.1 
22  5 

Wabash  clay  (WC)  
Sharkey  clay  (SC) 

1.0 
0  2 

0.4 
0  2 

3.3 
2  0 

4.2 
5  2 

50.3 
44  6 

40.1 
47  4 

Since  sand  (silica)  constitutes  much  of  the  load  of  streams  and  the 
sand  grains  are  very  durable  and  therefore  relatively  large,  it  follows 
that  sand  is  quickly  deposited  and  much  of  the  sandy  load  is  found  in 
the  soils  of  the  front  lands.  These  soils  are,  therefore,  siliceous  as  com- 
pared with  those  of  the  back  lands.  Sands,  sandy  loams  and  loams 
are  the  prevalent  types  of  front  lands.  The  percentages  of  lime,  potash 
and  phosphoric  acid  in  the  front  land  soils  are  relatively  low,  although 
many  of  these  soils  have  enough  mineral  plant  food  for  crop  use,  and 
their  excellent  drainage  on  the  whole  makes  them  productive.  On  the 
other  hand,  the  clays  and  silts  of  the  back  lands  show  high  percentages 
of  these  plant  foods  in  part  because  they  are  fine  grained,  a  relation 
between  texture  and  composition  that  has  been  before  noted. 

/This  typical  variation  in  composition  is  illustrated  in  Fig.  126,  and  the  accom- 
panying table.  The  map  shows  the  soils  on  a  portion  of  the  flood  plain  of  the  Kansas 
River.  The  belt  of  sandy  soils  here  is  relatively  broader  than  on  the  flood  plains 
of  larger  rivers,  a  comparison  that  usually  holds  for  the  flood  plains  of  smaller  rivers, 

1  Soil  Survey  of  East  Carroll  Parish,  La.,  U.  S.  Bureau  of  Soils. 


FLOOD  PLAINS 


155 


The  results  are  given  in  pounds  per  acre,  assuming  2,000,000  pounds  per  acre,  sur- 
face soil  7  inches  deep. 


Phosphoric 
acid. 

Potash. 

Lime. 

Osage  very  fine  sand  (OS) 

440 

40200 

15600 

Osage  very  fine  sandy  loam  (OF) 

820 

39200 

15000 

Osage  silty  clay  loam  (OL)        .    

1,220 

41  200 

16200 

FIG.  126. — Soils  on  the 
Kansas  River,  Kansas. 


The  surface  drainage  of  front-land  soils  is  fair  or  good  because  of 
the  slopes  and  the  underdrainage  is  helped  by  the  porous  textures.  On 
the  other  hand,  the  drainage  of  the  back  lands, 
both  surface  and  subsurface,  is  retarded  by  the 
level  slopes  and  the  relatively  impervious  fine- 
grained clays  and  silts  that  are  characteristic. 
A  feature  growing  out  of  the  retarded  drainage 
of  the  back  lands  is  the  high  humus  content  of 
many  of  these  soils.  It  has  been  noted,  page 
79,  that  vegetable  matter  often  accumulates 
under  such  conditions  of  retarded  drainage 

and  such  soils  are  likely  to  show  a  high  nitrogen  content.  The  humus 
and  nitrogen  account  in  part  for  the  high  fertility  of  these  back-land  soils 
when  they  are  well  drained. 

Variability  of  Alluvial  Soils. — While  all  alluvial  soils  are  character- 
istically variable,  yet,  as  a  rule,  the  soils  along  large  rivers,  especially 
in  the  lower  reaches,  are  less  variable  than  those  along  the  upper  reaches 
and,  other  things  being  equal,  the  soils  along  small  streams  are  more 
variable  than  those  along  large  streams.  Along  the  lower  portions  of 
large  streams  the  currents  are  less  variable  because  the  slopes  are 
gentle  and,  therefore,  there  is  less  variation  in  load  that  is  deposited. 
Furthermore,  the  river's  load  is  well  mixed  by  its  long  journey  and  there 
is  less  local  variation  of  materials.  Thus,  along  the  lower  Mississippi 
one  can  be  fairly  certain  that  the  river  will  be  bordered  by  loamy  front 
lands  with  wide  belts  of  silts  and  clays  on  the  back  lands.  The  soils 
along  the  upper  reaches  of  a  river  are  often  variable  or  "  patchy."  The 
slopes  here  are  steeper  than  in  the  lower  reaches  so  that  there  is  greater 
variation  in  velocities  between  high  and  low  water  and,  in  consequence, 
there  is  a  variety  of  materials  deposited.  Furthermore  the  load,  as  a 
rule,  has  had  a  shorter  journey  and  is  less  thoroughly  mixed. 

Fig.  127  shows  the  variable  soils  in  the  upper  part  of  the  Sacramento  River  in 
California.  The  river  descends  from  the  mountains  with  a  high  velocity  and  con- 


156 


CLASSES  OF  ALLUVIAL  DEPOSITS 


sequently  carries  a  heavy  load,  including  gravel.  When  the  river  reaches  the  lower 
more  flat  country,  the  heavy  gravels  are  dropped  making  large  areas  of  gravelly 
soils.  The  variable  velocities  here  explain  the  range  of  soils  from  gravels  to  silts,  a 
complex  arrangement  in  marked  contrast  to  the  simpler  arrangement  in  the  lower 
parts  of  the  same  river. 

Alluvial  soils  are  especially  notable  for  sudden  variations  between 
soil  and  subsoil.  That  alluvial  deposits  vary  vertically,  as  well  as 
laterally,  has  been  noted  before,  Fig.  120.  A  strong  current  may  deposit 
coarse  materials  during  high  water  and  weaker  currents  of  falling 

water    may  deposit  finer    ma- 

L 

terials.  Again,  slow  currents 
may  cover  coarse  deposits  of 
former  times  with  fine  materials. 
In  general,  stream  deposits  tend 
to  become  finer  from  the  bottom 
up,  and  sand  and  gravel  are 
likely  to  be  found  in  the  lower 
sections  of  alluvial  deposits  and 

sands,   silts   and    clays  in   the 
FIG.  127. — Soils   of   the   rapid  Sacramento  ,.  ,.,    *     ,     ., 

River,  Cal.  In  the  profile  above,  the  uPPer  sections,  although  there 
area  below  the  words  "soil  map"  is  shown  are  many  exceptions  to  the  rule, 
on  the  map  below.  The  river  flows  from  Very  often  a  field,  say  of  loam, 
left  to  right.  (U.  S.  Bureau  of  Soils).  wiH  be  well  drained  in  some 

places  and    poorly  drained    in 

others  and  the  soil  auger  will  show  differences  in  subsoils  as  the 
cause.  As  in  soils,  variations  in  subsoils  are  more  common  along 
small  streams  than  large  ones  and  in  the  upper  rather  than  in  the 
lower  courses  of  rivers. 


GRAVELLY   LOAMc^o        SKTJ.OAM   ^^          F'NELOAM 
LOAM    :£_5~:=  RIVER  WA3H  (.<"/£? 


Flood  Plains  and  Valleys 

Flood  plains  occur  most  typically  in  the  lower,  rather  than  in  the 
upper  parts  of  valleys,  for  here  the  valley  is  usually  wider  and  more 
likely  to  be  flat  so  as  readily  to  be  overflowed  and  built  up.  If,  as  is  the 
case  in  some  places,  the  rocks  in  the  lower  part  of  a  valley  are  more  espe- 
cially resistant,  the  valley  may  actually  be  narrower  in  its  lower  part  as, 
for  example,  the  Connecticut  River,  the  lower  valley  of  which  is  narrow 
and  gorge-like  and  the  upper  valley  is  wider  and  the  river  is  fringed 
by  alluvial  lands. 

As  erosion  proceeds  the  river  valley  is  widened  up  stream  and  the 


FLOOD  PLAINS  AND  VALLEYS 


157 


FIG.  128. — A  river  meandering  in  its  flood 
plain.  The  meanders  are  widening  the 
valley,  Canada.  (Canadian  Geological 
Survey.) 


areas  of  flood  plains  also  extend  up  stream.     A  feature  of  flood  plains 

is  that  they  are  ordinarily  bounded  on  either  side  by  relatively  steep 

slopes.     These  slopes  may  be  so  steep  as  to  form  bluffs  if  the  valley  has 

been  widened  by  lateral  cutting  of  the  stream  as  shown  in  Fig.  128  or 

if  the  rocks   along   the  stream 

are    somewhat   resistant.      On- 

the  other  hand,  when  the  rocks 

in  a  stream  valley  are  weak  and 

are    easily   eroded,  the  valley 

sides  may  have  slopes  so  gentle 

that  it  is  difficult  to  determine 

where  the  flood  plain  ends  and 

the  valley  sides  begin. 

Flood  Plain  Meanders. — A 
river  flowing  through  a  flood 
plain  seldom  holds  a  straight 
course  and  if  its  course  is 

straight  it  will  soon  become  curved.  The  primary  reasons  for  such 
common  meandering  habits  are  (1)  that  such  streams  usually  have 
rather  low  velocity  and  are  easily  turned  aside  and,  (2)  furthermore, 
the  incoherent  materials  of  flood  plains  offer  little  resistance  to 
changes  in  the  stream  courses.  (3)  The  banks  here  of  sand,  there 
of  clay,  may  offer  unequal  resistance  to  the  currents  and  thus  promote 
changes  in  the  stream's  course.  Sunken  boats  or  lodged  timber  have 
been  known  to  start  meanders,  but  very  often  a  river  will  cut  more  on 

one  bank  than  on  the  other  for 
no  apparent  reason.  A  stream 
in  a  meander  usually  under- 
cuts its  bank  beneath  the 
surface  of  the  water  so  that 
often  acres  of  land  will  sud- 

FIG.  129.— Revetment  in  the  Missouri  River  denl7  slumP  into  the  water 
to  prevent  undercutting.  The  river  is  flow-  along  a  long,  narrow  zone, 
ing  from  left  to  right.  Such  undercutting  is  some- 

times prevented  by  revet- 
ments, Fig.  129,  which  deflect  the  current  from  the  bank.  Flood 
plain  meanders  practically  always  characterize  a  river  flowing  through 
a  flood  plain.  They  differ  from  incised  meanders,  page  139,  in  that  the 
valley  follows  incised  meanders  while  flood  plain  meanders  are  quite 
independent  of  their  valley. 


158 


CLASSES  OF  ALLUVIAL  DEPOSITS 


FIG.  130. — Diagram  to  illustrate  the  out- 
ward and  down-stream  movements  of 
meanders  (1  to  4).  Arrows  show  the 
position  of  the  rapid  current.  Dotted 
areas  are  those  passed  over  by  the  river. 
At  the  right  the  meanders  have  been 
converted  into  ox-bow  lakes. 


Development  of  Meanders.— The  work  of  a  stream  in  a  flood  plain 

meander  is  much  as  is  an  incised  meander  except,  of  course,  the  changes 

are  much  more  rapid  in  the  former.  The  rapid  current,  Fig.  130,  im- 
pinging against  the  banks  at  A 
and  B  erodes  at  these  places 
more  than  elsewhere  so  that  the 
meander  not  only  moves  lateral- 
ly but  down  stream  as  well. 
Successive  positions  of  the 
meander,  due  to  these  combined 
lateral  and  down-stream  move- 
ments, are  shown  in  the  diagram- 
Often  the  meanders  approach 
so  closely  to  each  other  that, 
during  high  water,  the  stream 
breaks  through  the  narrowed 

neck  leaving  the  former  meander  as  a  curved  "  ox-bow  "  lake.  When  this 

happens  the  river  shortens  its  course  and  the  current  is  swifter,  so  that  a 

new  meander  is  likely  to  be 

formed  because  of  the  shifting 

of  currents. 

When  a  meander  is  thus 

cut  off  from  the  main  stream, 

it  is  filled  at  the  ends,  changes 

into    a    lake,  the    lake    later 

reaches  the  swamp  stage  and 

in  time  the   swamp   becomes 

filled  and  can  often  be  traced 

by  the  peculiar  crescent-shaped 

areas  of  soils,  Fig.  131.     Fig. 

132  shows  an  interesting  ex- 
ample   of    vanishing    ox-bow 

lakes.      The    upper    lake    is 

evidently    the    younger    and 

the  former  meander   can   be 

traced  for  much  of  its  course. 

The    lower    lake    is     but    a 

remnant  of  a  former  ox-bow 

lake.       The    lakes    are    being    filled    with    fine    materials,    mostly 

clays. 


1  Mile 


LOAM  :•&££» 


MUCK  SOILS  OF 
FILLED   LAKES 


FIG.  131. — Former  ox-bow  lakes  shown  by 
muck  soils,  Missouri.  (U.  S.  Bureau  of 
Soils.) 


FLOOD  PLAINS  AND  VALLEYS 


159 


-lirl-J. 


im 

ill 


YAZOO  SANDY  LOAM 
WABASH  CLAY 


SWAMP  ^  j^ 


YAZOO  LOAM 
SHARKEY  CLAY 


Deposition  by  Meanders. — Not  only  does  a  meander  erode  as  it 

moves  outward  and  down  valley  but  it  deposits  as  well.     The  -rapid 

current  is  on  the  outside  of  the 

meander  and,  in  consequence,  most 

of  the  cutting  is  done  here.     On 

the  other  hand,  the  water  on  the 

inside  of  a  meander  is  more  quiet 

and  consequently  deposition  usual- 
ly occurs  here.      This  deposition 

on  the  inner  portions  of  a  meander 

is  well  shown  in  Fig.  133,  which 

shows  the   meander  building    on 

the  inner  side  (left)  and  cutting  on 

the  outer  side  (right).     It  will  be 

seen  that,  as   a   meander   moves 

down  valley,  it  both  removes  and 

deposits  materials  and,  when  it  is 

remembered  that  there  are  many 

meanders    often    following    each 

other  in  the  same  stream,  it  will 

be  evident  that  a  vast  amount  of 

material    is  moved  in  this  way. 

Moreover  a  long  stretch  of  a  river 

with  its  many  meanders  will  swing  from  one  side   of  its  valley  to  the 

Other,  thus  widening  the  valley.     The  lower  Mississippi,  for  example, 

has  undoubtedly  widened  its 
valley  in  this  manner. 

An  example  of  the  relation 
of  soils  to  meanders  along  the 
lower  Mississippi  is  shown  in 
Fig.  134.  The  river  formerly 
followed  a  course  approximate- 
ly along  the  line  of  arrows.  The 
meander  has  moved  laterally 
and  down  streamward  leav- 
ing in  its  wake  several  square 
miles  of  newly  made  soils. 
The  Settlement  of  Flood  Plains. — Flood  plains  are  usually  first 

occupied  in  the  settlement  of  a  region  both  because  of  their  productive- 
ness and  because  of  their  accessibility.     The  natural  levee  is  higher  and 


FIG.  132.— Partly  filled  ox-bow  lakes,  La. 
(U.  S.  Bureau  of  Soils.) 


FIG.  133. — The  Missouri  River  depositimg 
sediment  on  the  inside  of  a  meander  at 
the  left. 


160 


CLASSES  OF  ALLUVIAL  DEPOSITS 


usually  better  drained  than  the  back  lands  and  this  difference  often 
leads  to  crop  differentiation  on  front  and  back  lands  and,  indeed 
these  two  divisions  are  often  so  clearly  marked  by  crops  that  the  crops 

map  the  soil  types,  Fig.  135. 
Very  often  the  front  lands  are 
cleared  while  the  back  lands  are 
in  forest  and  here  again  the 
vegetation  often  marks  the  soils, 
the  line  of  timber  being  near 
the  boundary  between  the  front 
and  back  lands.  Because  of  this 
difference  in  soils,  land  lines  on 
flood  plains  are  often  roughly 
perpendicular  to  the  river  so 
that  the  farm  or  plantation  will 
include  both  soil  types. 


FIG.  134. — Showing  the  deposition  of  soils 
(dotted  areas)  as  a  meander  of  the  Mis- 
sissippi moves  down  stream.  The  ar- 
rows indicate  approximately  the  former 
position  of  the  river,  Louisiana.  (FL) 
front  land;  (BL)  back  land. 


The  Mississippi  Flood  Plain.— 

By  far  the  largest  flood  plain  in 
North  America  and  one  of  the 
largest  in  the  world  is  that  of 
the  Mississippi  River.  The  main 

part  of  this  flood  plain  extends  from  Cairo  at  the  junction  of  the 
Ohio  to  the  Gulf  of  Mexico.  This  area  varies  from  30  to  60  miles 
in  width  and  has  a  length  of  about  600  miles,  Fig.  136.  Through 
this  alluvial  plain  the  great 
river  meanders  for  a  distance 
of  about  1100  miles  or  about 
half  that  distance  by  air  line. 
On, either  side  for  much  of  the 
distance  are  bounding  bluffs  of 
varying  heights.  The  vast 
alluvial  plain  is  really  a  com- 
plex of  the  flood  plain  of  the  FlG>  135 .—The  front  lands  (loams)  are  in 
Mississippi  and  its  tributaries  alfalfa;  the  back  lands  (clay  loams)  are 
which  usually  flow  roughly  in  wheat,  Kansas, 
parallel  with  the  main  river 

before  entering  it.  The  back  lands  especially  contain  a  network  of 
small  streams  each  fringed  with  its  own  natural  levee.  Many  low 
ridges  which  were  built  as  natural  levees  mark  the  courses  of  streams 
that  have  disappeared,  Fig.  137.  The  population  and  cultivated  lands 


ALLUVIAL  TERRACES 


161 


are  mainly  located  in  a  relatively  narrow  belt  along  the  front  lands 

The  back  lands  of    the   Mississippi  River 

are    largely    in    timber    and    yield    large  M  0§ 

amounts  of   lumber.     Their  drainage   and 

utilization  is  one  of  the  large  problems  in 

the  farm  development  of  the  United  States. 

ARK. 

Alluvial  Terraces 

Many   stream   valleys    are    fringed   by 

strips  of   level   land   which   lie   above  the 

stream    and     flood    plain    and    are    called 

terraces,  Fig.  138.     The  more  or  less  level 

terrace  surface  descends  toward  the  stream 

by  a  slope  termed  the  terrace  face,  which 

is  often   relatively  abrupt,  but  may  occa- 
sionally be  gentle.     Not  infrequently  there 

are  several  terraces  rising   one    above  the 

other  like  a  huge,    rude   stairway   on   one 

or  both  of  the  valley  sides.     Terraces  are 

usually  narrow,  seldom  being  more  than  a 

few  miles   in   width   at  most.      They   are 

seldom  continuous  for   long  distances  and 

they  lie  at  various  heights  above  the  stream,  FlG>  i36.— The  alluvial  plain 

the  lowest  terrace  sometimes    being    inun-     and  delta  of  the  Mississippi. 

dated   by  high  water   while  some  terraces 

are  hundreds  of  feet  above 
the  valley  .bottom.  Terraces 
are  frequently  termed  "  sec- 
ond bottom,"  "third  bot- 
tom," etc.,  to  distinguish 
them  from  the  flood  plain  or 
"  first  bottom." 

Origin. — Terrace  materials 

FIG.  137. — Low,  loamy  ridges  (dotted)  built 
by  former  streams  on  a  flood  plain,  La. 
(U.  S.  Bureau  of  Soils.) 


5  Mil 


are  usually  stratified  and 
show  all  the  indications  of 
water  deposition.  They  are 
remnants  of  flood  plains  most 

of  which  have  been  eroded  and  they,  therefore,  represent  two  phases 
of  stream  work.    First,  the  stream  must  erode  its  valley  and,  then; 


162  CLASSES  OF  ALLUVIAL  DEPOSITS 

owing  to  some  change  in  conditions  the  stream  must  fill  its  valley  to 
a  considerable  depth  with  alluvial  materials.  Subsequently,  changing 
its  habits  from  aggrading  to  degrading,  the  stream  cuts  down  through 
the  alluvial  materials,  leaving  more  or  less  of  the  old  flood  plain  standing 
as  alluvial  terraces.  %  The  successive  processes  are  illustrated  in  Fig.  139. 
Some  of  the  factors  which  cause  changes  in  stream  habits  whereby 
terraces  are  formed  are  as  follows:  (1)  A  tilting  and  uplift  of  the  land 
may  make  the  grade  of  a  stream  higher,  thereby  giving  it  greater  velocity 
and  consequently  sufficient  energy  to  cut  instead  of  deposit.  (2) 
If  there  is  an  increase  in  volume  without  a  corresponding  increase  of 


FIG.  138. — Alluvial  Terraces,  Washington.     Note  the  level  tops  and  steep  fronts 
of  some  of  the  terraces.     (Russell,  U.  S.  Geological  Survey.) 

load,  the  stream  may  change  from  aggrading  to  degrading  activities; 
such  a  result  might  result  through  a  change  from  dry  to  moist  climate. 
(3)  If  from  any  cause,  a  stream's  load  is  decreased  while  other  factors 
remain  practically  constant,  the  stream's  energy  expended  in  carrying 
the  load  is  released  and  may  be  applied  to  cutting  its  bed.  The  forma- 
tion of  terraces  was  especially  active  at  the  close  of  the  Glacial  Period, 
when  conditions  were  particularly  favorable  for  terraces.  During  this 
period  vast  glaciers  covered  much  of  North  America  and  Europe, 
Figs.  170  and  171,  and,  when, the  ice  melted,  the  heavily  laden  streams 
flowing  from  the  glaciers  filled  their  valleys  to  depths  of  scores  and 
hundreds  of  feet.  Later  the  load  was  reduced  and  perhaps  the  land 
was  tilted  as  the  weight  of  the  ice  was  removed  and,  as  a  result,  terraces 
of  glacial  materials  extend  along  many  streams  far  beyond  the  limits 


ALLUVIAL  TERRACES 


163 


of  glaciation.     This  period  is  sometimes  called  the  "  terrace  epoch  " 
because  of  its  many  associated  terraces. 

It  has  been  noted  that  terraces  often  rise  one  above  the  other  like 
rude  stairs.  This  means,  in  general,  that 
there  were  as  many  successive  cuttings  as 
there  are  terraces.  Such  a  series  of  terraces 
could  be  produced  by  successive  uplifts  of 
the  land.  For  example,  one  uplift  would 
cause  the  stream  to  erode  its  channel  and 
leave  the  former  flood  plain  remnants  stand- 
ing as  terraces.  If  the  land  remained  quiet 
for  a  sufficient  time  the  stream  might 
meander  and  destroy  much  of  the  terraces. 
A  second  uplift  would  result  in  another 
erosion  of  the  second  flood  plain  again  leav- 
ing fragments  of  this  flood  plain  as  terraces. 
Thus  each  elevation  might  result  in  a 
terrace.  Again  there  might  be  a  variation 
in  volume  whereby  a  period  of  stream 
building  might  be  followed  by  a  period  of 
stream  cutting.  Under  such  conditions  we 
might  find  as  many  terraces  as  there  were 
variations  in  volume.  Finally,  it  may  be 
noted  that  terraces  might  be  made  by  the 
swinging  from  side  to  side  of  a  series  of 
meanders. 

In  resume  it  may  be  said  that  terraces 
are  fewer  due  to  destruction  than  action. 
For  this  reason  terraces  are  discontinuous, 
for  they  have  been  completely  destroyed 
in  places;  they  seldom  continue  for  more 
than  a  few  miles  at  most. 

Terrace  Soils. — In  general,  the  soils 
of  terraces  are  likely  to  be  coarser  than 
those  of  the  adjacent  flood  plains.  This  is 
especially  true  of  glacial  terraces,  which  have 
been  mentioned  before,  since  the  streams 
which  deposited  these  terraces  usually  carried  coarser  materials  than 
the  present  streams.  Terrace  soils  are  commonly  better  drained  than 
those  of  flood  plains,  both  because  of  the  higher  elevation  of  terrace 


FIG.  139. — Diagram  to  illus- 
trate (A)  valley  cutting;  (B) 
valley  filling;  (C)  terrace 
making. 


164 


CLASSES  OF  ALLUVIAL  DEPOSITS 


soils,  and  their  usually  coarser  texture.  In  general,  they  contain  less 
humus  than  flood-plain  soils.  A  common  textural  characteristic  of 
terrace  soils  is  that  they  become  coarser  downward.  That  is,  the  sub- 
soil and  the  materials  below  the  subsoil  are  frequently  gravelly  while 
the  soil  may  be  sandy  or  loamy.  This  is  explained  by  the  fact  that,  as 
the  original  flood  plain  from  which  the  terrace  was  cut  was  built  up,  the 
gradient  of  the  stream  became  less  and  it,  therefore,  carried  and  deposited 
finer  materials. 

Terraces  are  older  than  the  adjacent  flood  plains  and  higher  terraces 
are  older  than  lower  ones.  Consequently,  the  soils  of  terraces  are  rela- 
tively somewhat  weathered  as  a  rule.  Old  terraces  may  be  so  eroded 
as  to  have  lost  all  traces  of  their  former  level  surface  and  are  only 
recognizable  by  a  thin  belt  of  characteristic  soils.  Fig.  140  shows  the 

level  surface  of  a  young  terrace 
bordered  by  an  older  eroded 
terrace.  Old  terrace  soils  may 
be  so  thin  that  they  are  under- 
lain by  a  residual  subsoil.  Ter- 
races, the  upper  parts  of  which 
are  in  contact  with  uplands, 
show  gradations  in  soils  from 
terrace  to  upland  soils  and, 
where  there  is  a  marked  slope 
between  terrace  and  upland, 
there  is  often  a  belt  of  soils 
which  have  been  washed  down 
from  the  upland.  If  the  terrace 

soils  resemble  the  upland  soils,  as  is  the  case  in  some  parts  of  the  Coastal 
Plain,  there  is  obvious  difficulty  in  distinguishing  between  the  two  soils 
and,  again,  terrace  soils  may  be  weathered  so  as  to  resemble  the  upland 
soils  rather  closely.  A  contrast  sometimes  to  be  observed  between 
flood-plain  soil  and  the  older  associated  terrace  soils  is  in  the  subsoils. 
It  has  been  noted  that,  in  general,  subsoils  in  humid  climates  are  some- 
what heavier  than  the  soils  owing  to  the  action  of  ground  water  in  car- 
rying downward  the  finer  silts  and  clays.  This  action  requires  a  con- 
siderable time  and  older  terraces  may  show  very  distinct  heavy  subsoils 
while  younger  terraces  and  still  younger  flood  plains  may  show  but 
little  difference  between  the  soil  and  the  upper  subsoil. 


FIG. 140. — Diagram  to  illustrate  (A)  a  lower, 
smoother  young  terrace  and  (B)  an  upper, 
older  and  eroded  terrace. 


ALLUVIAL  SOILS  AND  STREAM   BASINS 


165 


Alluvial  Soils  and  Stream  Basins 

It  is  obvious  that  there  must  be  a  more  or  less  close  relationship 
between  stream  deposits  and  the  ultimate  source  of  the  alluvial  materials. 
For  example,  a  stream  draining  a  sandstone  basin  will  usually  deposit 
sandy  materials,  as  do  many  of  the  streams  flowing  through  sandy 
regions  of  the  Coastal  Plain.  It  must  be  remembered,  however,  that 
only  a  small  number  of  important  depositing  streams  drain  basins  that 
yield  fairly  homogeneous  materials.  Moreover,  it  is  usually  difficult 
to  identify  finer  stream  sediments  as  coming  from  a  given  locality,  for 
it  is  one  of  the  characteristics  of  stream  transportation  that  the  materials 
in  transit  are  thoroughly  mixed.  Indeed,  it  is  only  when  the  stream  load 
has  a  characteristic  color  or  composition  that  its  sediments  are  readily 
identified  as,  for  example,  the  characteristic  sediments  of  the  Red 
River,  hence  its  name.  Again,  the  fertility  of  river  soils  is  often  as 
much  a  matter  of  texture  as  of  com- 
position, and  texture  is  usually  more 
dependent  on  stream  sorting  than  on 
the  original  source  of  the  sediments. 

Examples. — Fine  examples  of  relations 
between  stream  basin  and  alluvial  soils 
are  found  in  Texas  and  adjoining  states, 
Fig.  141.  The  Red,  Brazos  and  Colorado 
Rivers  rise  in  regions  of  Permian  rocks 
which  include  much  red  sandstone  and 
shale.  Such  materials  color  the  alluvial 
soils  of  these  rivers  for  more  than  three 
hundred  miles  down  stream  from  the  red 
rocks  which  yield  the  typically  reddish 
soils  (Miller  series).  On  the  other  hand 
the  neighboring  Trinity  River  rises  and 
flows  for  a  considerable  distance  through 
calcareous  materials  which  yield  black, 

heavy  alluvial  soils.  The  Rio  Grande  farther  south  rises  in  the  semi-arid  regions 
of  New  Mexico  and  flows  through  regions  of  scant  rainfall,  and  as  a  conseqence, 
its  soils  are  but  little  leached,  are  light  colored  and  are  very  Jiigh  in  lime. 

Glacial  deposits  cover  much  of  the  upper  Mississippi  Basin,  and  their 
fine-grained  materials  yield  the  productive  alluvial  soils  of  that  region. 
The  high  productivity  of  the  lower  Mississippi  soils  is  doubtless  due,  in 
some  measure,  to  the  admixture  of  comparatively  unweathered  glacial 
materials  which  have  been  carried  by  the  river  for  hundreds  of  miles. 
It  is  sufficient  for  our  purposes  to  state  that  the  glacial  materials  are 


FIG.  141. — The  Red,  Brazos  and  Colo- 
rado rivers  rise  in  regions  of  red 
Permian  rocks  (A).  The  Trinity 
River  rises  in  a  belt  of  chalks  and 
marls  (B)  which  furnish  calcareous 
materials  to  this  river. 


166  CLASSES  OF  ALLUVIAL  DEPOSITS 

mainly  rock  "  ground  up  "  by  glaciers  so  that  the  materials  are  com- 
paratively fresh  and  unweathered.  We  find  somewhat  characteristic 
alluvial  soils  in  New  England,  where  the  rocks  are  mainly  granite  and 
gneiss.  Another  illustration  is  found  in  northwestern  Washington, 
where  alluvial  soils  from  the  glaciated  region  are  underlain  by  fine  sand 
while  the  alluvial  soils  from  regions  of  residual  soils  have  heavy  sub- 
soils. The  explanation  of  the  contrasting  subsoils  is  to  be  found  in  the 
fact  that,  when  the  former  extensive  glaciers  melted,  the  resulting  large 
volumes  of  water  carried  and  deposited  the  sands,  which  were  later 
covered  with  finer  silts  as  the  depositing  waters  later  lost  their  volume. 
Streams  often  carry  alluvial  materials  down  stream  into  regions  of 
different  soils  so  that  the  soils  of  one  region  may,  so  to  speak,  be  pro- 
jected as  long  tongues  into  other  regions.  For  example,  the  fine- 
grained soils  from  the  Piedmont  extend  as  narrow  alluvial  tracts  through 
the  sandy  Coastal  Plain,  and  the  projection  of  glacial  materials  as  allu- 
vial soils  far  southward  of  the  original  locations  has  been  noted  on 
preceding  pages. 

Deltas 

Deltas  are  built  where  streams  enter  relatively  quiet  water  wnere 
the  stream  velocity  is  suddenly  checked  and  thus  rapid  deposition  is 
brought  about.  Not  all  streams  built  deltas;  the  Niagara  River,  for 
example,  has  not  built  a  delta  into  Lake  Ontario,  for  the  river  carries 
little  sediment  as  it  flows  from  Lake  Erie.  Again,  if  there  are  strong 
tides,  waves  or  currents  at  the  mouth  of  a  stream  they  will  carry  away 
the  sediment  that  would  otherwise  be  built  into  a  delta.  Deltas  are, 
therefore,  the  "  triumph  of  river  deposition  over  wave  and  current 
destruction "  (Grabeau).  Because  waves,  tides  and  currents  are 
stronger  in  oceans  and  seas  than  in  lakes,  deltas  are  usually  more 
abundant  in  lakes,  although  the  larger  deltas  are  usually  built  into 
seas  because  here  they  are  built  by  the  larger  rivers.  The  Mississippi 
delta,  for  example,  while  it  is  commonly  said  to  be  advancing  at  the 
rate  of  about  300  feet  a  year,  is  nevertheless  suffering  from  con- 
siderable erosion,  as  shown  in  Fig.  142. 

If  a  coast  is  rather  smooth,. the  delta  often  projects  beyond  the  coast 
line,  but  when  a  stream  empties  into  a  bay,  as  the  Susquehanna  into 
Chesapeake  Bay,  the  delta  simply  fills  the  bay  and  the  delta  outline 
is  more  or  less  governed  by  the  shape  of  the  bay,  as  shown  in  Fig.  143, 
where  the  Payallup  River  is  building  its  delta  into  Commencement 


DELTAS 


167 


Bay,  an  extension  of  Puget 
Sound.  Here,  as  usual,  the 
coarse  materials  are  deposited 
near  the  mouth  of  the  stream, 
the  finer  materials  are  carried 
into  more  quiet  water  and  there 
deposited.  The  marsh  near  the 
lower  part  of  the  delta  will  be 
built  up  into  arable  land  and 
the  water  near  the  delta  will 
become  shallow  and  then  give 
way  to  marsh. 

Growth  of  Deltas. — In  con- 
sidering the  formation  of  deltas, 
a  simple,  somewhat  ideal,  case 
will  be  taken.  Suppose  the 
stream,  Fig.  144 A,  carries  a 
heavy  load  of  sediment.  When 
the  stream  reaches  quiet  water, 
the  bulk  of  its  load,  especially 
the  coarser  load,  will  be  dropped 
near  the  stream  mouth  while 
the  finer  materials  will  be  carried 


FINE  SANDY  LOAM 
SILT  LOAM 
SALT  MARSH 


FIG.  143. — Soils  of  the  Puyallup  River  delta, 
Oregon.  The  river  is  building  its  delta 
into  Commencement  Bay.  (U.  S.  Bureau 
of  Soils.) 


FIG.  142. — Map  of  the  "passes"  of  the  lower 
Mississippi  delta  showing  the  areas  gained 
by  deposition  and  those  lost  by  wave  and 
current  erosion.  The  black  areas  show 
the  area  of  the  delta  in  1872,  the  lined 
areas,  the  regions  of  deposition  and  the 
dotted  areas,  the  regions  eroded.  (U.  S. 
Coast  and  Geodetic  Survey.) 


farther  out.  The  mass  of 
debris  first  deposited  tends 
•to  reduce  the  grade  of  the 
stream  and  makes  the  single 
channel  unable  to  carry  the 
water  and  sediment  so  the 
stream  escapes  through  distri- 
butaries, Fig.  U4B.  These 
distributaries  become  aggrad- 
ing streams  and  build  up 
natural  levees  and  back  lands 
much  the  same  as  streams  in 
a  flood  plain  and,  moreover, 
the  distributaries  themselves 
develop  smaller  distributaries. 
Between  the  growing  distrib- 
utaries are  delta  lakes,  Fig. 


168 


CLASSES  OF  ALLUVIAL  DEPOSITS 


144  B  and  C,  which  become  filled  with 
sediment  brought  by  overflows  and  the 
areas  of  which  become  lessened  by  the 
widening  of  the  natural  levees.  In  time 
the  lakes  are  mostly  filled  and  the  older 
parts  of  the  delta  are  built  above  sea  level 
as  a  fairly  continuous  land  area,  while  the 
processes  outlined  above  are  continued 
farther  seaward.  Thus  the  delta  in  its 
older  portions  is  built  into  a  low,  level 
plain,  interrupted  here  and  there  by 
deserted  natural  levees,  sluggish  streams 
and  shallow  delta  lakes,  while  beyond  to 
seaward  the  partly  completed  delta  is  in 
process  of  building. 

It  should  be  kept  in  mind  that  the 
simple,  ideal  conditions  stated  above 
seldom  apply  in  all  respects  to  any  delta 
because  of  the  many  complicating  factors, 
such  as  the  rising  or  sinking  of  the  coast, 
waves,  tides,  currents,  variations  in  the 

FIG.  144.— Diagram  to  illustrate  river  load  and  velocity,  and  perhaps  other 
le  stages  m  delta  building.     factors>     The  idea  to  be  emphasized  in 
the  usual  growth  of  deltas  is  their  ulti- 
mate conversion  into  delta  plains, 

which  are  practically  extensions 

of  the  flood    plains  above,  and 

no  definite  line    of   division  be- 
tween the   delta  plain  and  the 

flood  plain  above   can  be  drawn. 

The  great  Mississippi  Delta  is 

a  case  in  point,  Fig.  145.   Locally 

the  part  projecting  below  New 

Orleans  is  called  the  delta,  while 

some  geologists  would  place  the 

head  of  the  delta  some  250  miles 

up  the  river  where  the  first  distri-  ,, 

u    ,     .                             «.      0             .  FIG.  145.— Map  of  the   Mississippi  delta 

butanes  are  given  off.     Some  of  showing  the  distributaries  from  the  Red 

the  world's  large  deltas  are  built  River  southward, 

by  two  or  more  rivers.     Holland 


DELTAS 


169 


FIG.  146.— Combined  delta  of  the 
Brahmaputra  and  Ganges  rivers. 
(After  Geikie.) 


is  a  classical  example  of  delta  reclamation;  it  is  located  in  part  on 
the  combined  deltas  of  the  Rhine,  Meuse,  Sambre  and  Scheldt.  The 
combined  deltas  of  the  Ganges  and  Brahmaputra  in  India  are  estimated 
to  cover  between  50,000  and  60,000 
square  miles.  The  intricate  network 
of  depositing  distributaries  of  these 
rivers  is  shown  in  Fig.  146. 

Classes  of  Deltas. — There  are  two 
classes  of  deltas  that  possess  such 
different  geological  and  agricultural 
features  that  they  deserve  separate 
descriptions.  First  are  the  deltas  of 
slow  streams  with  low  gradients  like 
the  Mississippi  River;  these  deltas 
have  been  considered  in  the  fore- 
going paragraphs.  Second  are  deltas 
of  rapid,  high-grade  streams.  Deltas 

of  rapid  streams  are  necessarily  built  of  relatively  coarse  materials 
because  the  constructing  streams  can  carry  gravel  as  well  as  sand 
and  silt.  The  heavy  load  falling  near  the  stream's  mouth  is  built 
into  a  level-topped  delta  with  a  steep  front,  steep  because  the 
coarser  materials  will  lie  at  rest  at  a  higher  angle  than  finer  materials. 
The  silt,  fine  sand  and  clay  are  spread  out  beyond  the  delta  front.  Such 

deltas  are  especially  charac- 
teristic of  lakes  and  bays 
surrounded  by  somewhat  high 
land  with  streams  flowing 
down  considerable  slopes. 
Many  such  lakes  of  large  ex- 
tent formerly  existed  in  the 
Great  Basin  and  most  of  these 
lakes  have  been  wholly  or 
partially  drained  and  their  dry 
deltas  now  stand  forth  on  the 
valley  sides  much  as  when 
they  were  built.  The  former 
Lake  Bonne ville,  Fig.  233,  has 

many  such  deltas  of  considerable  area.  Typically,  such  deltas  have 
sandy  or  gravelly  soils  in  their  upper  portions  and  silty  soils  on  the 
lower  slopes  leading  away  from  the  deltas  into  the  old  lake  bottoms. 


FIG.  147. — Seward,  Alaska.  The  town  is  lo- 
cated on  a  delta  built  by  a  rapid  stream. 
(U,  S,  Geological  Survey.) 


170 


CLASSES  OF  ALLUVIAL  DEPOSITS 


These  delta  soils  are  of  especial  economic  importance  around  the 
eastern  margins  of  the  Great  Basin,  for  irrigation  water  can  often  be 
obtained  here. 

The  old  delta  of  the  American  Fork  River  in  Utah  is  an  excellent  example.  When 
the  Great  Basin  was  in  part  occupied  by  Lake  Bonneville  the  river,  a  rapid  stream 
heavily  loaded  with  debris,  built  a  large  delta  or  rather  a  series  of  deltas  at  different 
levels  of  the  lake.  This  delta,  including  several  thousand  acres,  projects  into  the 
valley  as  a  prominent  topographic  feature  and  the  American  Fork  River  and  Dry 

Creek  have  now  cut  their  channels  in 
places  to  the  base  of  the  delta;  other- 
wise, the  prominent  deltas  remain 
much  as  they  were  when  the  lake 
waters  were  drained  away.  The  soils 
shown  in  I  ig.  148  have  the  common 
arrangement  of  gravelly  soils  in  the 
main  body  of  the  delta  with  finer- 
textured  soils  about  the  delta  margin. 
The  delta  top  heading  at  the  stream 
canyons  is  easily  irrigated.  The  small 
areas  of  clays  are  believed  to  have 
been  washed  down  into  depressions 
since  the  lake  waters  withdrew. 

Much  of  the  same  kind  of  deltas 
are  found  where  there  were  formerly 
lakes  due  to  glacial  action ;  these  deltas 


GRAVELLY  LOAMS  °o_ooo 

SANDY  LOAMS  AND  SANDS  V/'/.V*. 

FIG.  148.— The  soils  of  the  old  delta  of 
the  American  Fork  River,  Utah.  (U.  S. 
Bureau  of  Soils.) 


will  be  considered  in   a   later  chapter. 

Such  a  delta  is  shown  in  Fig.  229.  These  exposed  "  dry  deltas,"  while  of  com- 
paratively small  areas,  are  of  greater  agricultural  interest  than  many  larger 
deltas  of  the  present,  since  the  latter  are  mostly,  submerged  and  only  a  small  part 
of  their  areas  are  available  for  cultivation. 


Delta  Materials. — All  large  deltas  are  built  by  large  rivers,  although 
not  all  large  rivers  build  deltas.  Large  rivers  and  all  rivers  with  low 
gradients  have  relatively  slow  currents  in  the  lower  courses  where  the 
deltas  are  built.  Their  deltas  are,  therefore,  mainly  composed  of  fine 
materials  such  as  clay,  silt  and  very  fine  sand  and  their  available  soils 
are,  therefore,  heavy.  Such  deltas  are  important  both  because  of  their 
extent  and  the  general  fertility  of  their  soils.  In  contrast  with  the 
deltas  of  extinct  lakes,  only  the  tops  of  present  deltas  are  exposed  to 
yield  soils  since  the  rest  of  the  delta  is  submerged.  The  fineness  of 
Mississippi  Delta  materials  is  indicated  by  the  following  table,  which 
shows  the  mechanical  analysis  of  a  composite  sample  selected  from 
235  samples  of  the  delta  materials: 


ALLUVIAL  FANS  AND  CONES 


171 


Fine  gravel 
(2  to  1  mm.). 

Coarse  sand 
(1  to  0.5 
mm.). 

Medium  sand 
(0.5  to  0.25 
mm.). 

Fine  sand 
(0.25  to  0.1 
mm.). 

Very  fine 
sand 
(0.1  to  0.05 
mm.). 

Silt  (0.05  to 
0.005mm.). 

Clay  (0:005 
mm.  or  less). 

•  3% 

•5% 

•2% 

6.5% 

28.2% 

51.2% 

13.0% 

SILTY  GLAY  LOAM 

SILT  LOAM        XXX     , 

CLAY  E-~EE 

SAND     '•  '•  •  •  •  •' 


*E.  W.  Shaw,  Professional  Paper,  No.  85,  U.  S.  Geological  Survey,  1914. 

Delta  soils  show  essentially  the  same  distribution  as  those  of  flood 
plains  since,  as  the  delta  is  built,  its  upper  portions  constitute  an  exten- 
sion of  the  flood  plain  which  becomes  longer  as  the  delta  is  extended. 
Delta  surfaces  are  level  except  where  they  are  interrupted  by  low  nat- 
ural levees  or  delta  lakes.  The  soils  are  rather  uniform,  although  the 
variation  from  coarse  to  fine  textures  is  seen  along  the  distributaries  as 
along  flood  plains.  Marginal 
portions  of  deltas  are  com- 
monly too  low  and  marshy  for 
cultivation  except  by  reclama- 
tion methods.  A  somewhat 
typical  soil  distribution  is  seen 
in  the  Rio  Grande  Delta,  which 
is  mainly  constructed  of  fine 
materials,  Fig.  149.  Along  the 
distributaries,  present  and  past, 
are  found  the  silty  clay  loams 
which  were  deposited  by  rela- 
tively rapid  currents  during  overflows,  a  relatively  light  soil  here, 
although  the  sand  content  is  very  low.  Clays  were  deposited 
between  the  distributaries  of  the  delta  and  along  its  margin  where 
the  river  currents  were  checked  by  the  Gulf.  The  clay  areas  contain 
some  unfilled  depressions  now  occupied  by  lakes.  The  silt  loams  are 
found  in  the  upper  and  newer  areas  of  the  delta  and  along  the  flood 
plain  although  here,  as  elsewhere,  there  is  no  sharp  distinction  between 
delta  and  flood  plain. 


FIG.  149. — Soils  of  a  part  of  the  Rio  Grande 
delta.     (U.  S.  Bureau  of  Soils.) 


Alluvial  Fans  and  Cones 

Alluvial  fans  and  cones  are  delta-like  deposits  built  on  the  land. 
Small  fans  and  cones  are  commonly  to  be  seen  on  pavements  and  in 
ditches  when  a  little  rill  descends  to  a  level  slope  and  there,  losing 


172 


CLASSES  OF  ALLUVIAL  DEPOSITS 


its  carrying  power,  drops  much  of  its  load  in  a  fan-like  or  cone-like 
shape.      Fans  and  cones  are  in  many  respects  much  like  deltas  built 


FIG.  150. — Above,  a  small  alluvial  fan,  front  view.     Below,  profile  view  of  a  small 
alluvial  fan,  California.     (Professor  C.  F.  Shaw,  University  of  California.) 

by  high-grade  streams  and,  in  fact,  some  fans  are  built  into  water 
and  form  deltas. 


ALLUVIAL  FANS  AND  CONES 


173 


make    steeper    slopes    near 
apex,  while   finer   materials 


Origin. — Fans  and  cones  typically  occur  at  abrupt  changes  in  stream 
slopes  as  where  mountains  abruptly  descend  to  plains.  The  mountain 
streams  lose  their  velocity  and  drop  their  coarser  materials  near  the 
mouths  of  their  gorges  and  these  coarse  materials  often  clog  the  stream 
so  that  it  divides  into  several  distributaries  much  as  those  of  deltas. 
The  coarser  materials  ordinarily 

the 
are 

carried  farther  down  the  fan 
and  built  into  more  gentle 
slopes.  The  distributary  chan- 
nels, often  termed  "  washes,"  are 
usually  dry  much  of  the  year. 
When  the  slopes  are  steep  the 

form  is  termed  a  cone  and  when  more  gentle,  a  fan,  although  both  forms 
merge  into  each  other;  fans  are  by  far -the  more  important  forms. 
Fans  and  cones  ordinarily  have  roughly  semicircular  outlines  with  lobed 
margins  and  lobes  of  which  as,  in  some  deltas,  extend  outwards  where 
distributaries  have  built  them. 


FIG.  151. — Diagram  to  illustrate  the  build- 
ing of  an  alluvial  fan. 


FIG.  152. — Diagram  to  show  simple  and  coalesced  alluvial  fans.  The  mountains 
on  the  right  are  higher  and  the  streams  have  built  the  larger  alluvial  fans  which 
have  grown  together  to  form  a  piedmont  plain. 

In  Fig.  151  the  high-grade  stream  reaches  lower  grades  at  A  where 
it  quickly  deposits  its  heavy  load  such  as  boulders,  stones  and  gravel; 
the  sand  is  carried  farther  out  into  zone  B  while  fine  sand,  silt  and  clay 
are  borne  still  further  from  the  apex  in  decreasing  quantities  so  that  the 
margin  is  usually  indistinct.  Often  the  feeding  stream  disappears  in 
the  apex  and  reappears  in  the  lower  portions.  Fans  and  cones  are 
largely  built  by  streams  in  flood.  Very  frequently  they  enlarge  until 


1?74  CLASSES  OF  ALLUVIAL  DEPOSITS 

they  unite  at  their  margins,  thus  forming  a  compound  alluvial  fan  or 
piedmont  plain,  Fig.  152.  ~ 

Favorable  Conditions. — Conditions  favorable  to  the  formation  of 
fans  and  cones  are  of  two  sorts,  topographic  and  climatic.  The  impor- 
tance of  an  abrupt  change  of  slope,  such  as  ordinarily  occurs  where  level 
plains  are  bordered  by  hills  or  mountains,  has  been  noted.  Especially 
important  areas  of  fans  in  North  America  are  at  the  western  base  of  the 
Sierra  Nevada  Mountains  in  California  and  at  the  eastern  base  of  the 
Rocky  Mountains.  Huge  fans  have  been  built  at  the  base  of  the  Hima- 
laya Mountains  in  India  and  at  the  base  of  the  Andes  in  Argentina. 
Small  fans  are  very  common  in  all  mountains  and  houses  are  often  built 
on  them.  (The  most  favorable  climatic  conditions  for  alluvial  fan 
building  are  found  in  dry  regions.  Dry  air  promotes  rapid  evaporation 
and  this  process  is  especially  effective  when  the  fan-building  stream 
divides  into  distributaries.  Evaporation  and  the  absorption  of  the 
water  by  the  underlying  porous  materials  of  the  fans  reduce  the  volume 
of  the  distributaries,  thus  aiding  deposition.  Furthermore,  the  rains  in 
this  region  are  often  torrential  and  feed  rushing  streams  of  high-carrying 
power  which  carry  to  the  fans  heavy  loads  of  coarse  materials.  Finally, 
fans  and  cones  built,  in  a  dry  climate  are  much  less  liable  to  destruction 
by  erosion  and  are  preserved  while,  on  the  other  hand,  these  forms  in 
humid  climates  are  likely  to  be  destroyed  or  at  least  poorly  preserved.^? 

Notable  Regions. — In  North  America  the  combination  of  high- 
'  sloped  streams  with  abrupt  changes  of  slope  and  dry  climate  is  found 
in  the  valley  of  California,  Fig.  153.  The  torrential  rainfall  in  the  Sierra 
Nevada  Mountains  is  carried  by  swift  mountain  streams  which  deposit 
their  load  in  united  alluvial  fans  along  the  base  of  the  mountains  as  a 
very  gently  sloping  piedmont  plain.  Here,  as  in  many  other  places, 
the  upper  portions  of  the  fans  grade  into  the  coarse  materials  (talus) 
which  creep  and  fall  down  the  steep  slopes.  The  mountains  on  the 
Western  side  of  this  valley  are  lower  and  have  less  rainfall  so  that  the 
fans  from  them  are"  less  extensive,  a  condition  illustrated  in  Fig.  152. 
A  favorable  agricultural  factor  in  the  fans  is  the  comparative  ease  with 
which  irrigation  water  is  led  from  the  building  streams  over  the  slopes 
of  the  fans.  In  some  regions  mountains  have  been  buried  for  hundreds 
of  feet  largely  by  the  fans  and  cones  which  have  been  built  around  them. 
i'The  High  Plains,  extending  from  the  Rocky  Mountains  eastward, 
are  believed  to  be  essentially  vast  coalesced  alluvial  fans  of  great  extent.1 

•    ^The   High   Plains    and  their  Utilization,  Willard  D.  Johnson,  23d  Annual 
Report,  U.  S.  G.  S.,  Part  4>  1899-1900. 


ALLUVIAL  FANS  AND  CONES 


175 


FIG.  153.— Northern  part  of  the  valley  of  California.     (U.  S.  Geological  Survey.) 


176 


CLASSES  OF  ALLUVIAL  DEPOSITS 


Much  of  this  area  is  so  level  that  water  stands  upon  the  surface,  but 
nevertheless  the  High  Plains  as  a  whole  rise  very  gently  hundreds  of 
feet  until  their  western  margin  is  nearly  a  mile  above  sea  level.  It  is 
believed  that  this*  plain  was  built  during  a  period  of  heavy  rainfall  by 
streams  which  flowed  eastward  from  the  Rocky  Mountain  region. 
Much  of  the  material  is  well  rounded,  thus  indicating  a  long  journey  by 
water  and  the  pebbles  are  composed  largely  of  crystalline  rocks  found 
to  the  westward  in  the  mountains.  In  structure  the  High  Plains  are 
composed  of  irregular  beds  of  gravel,  sand  and  silt,  much  as  are  found 
in  recent  alluvial  fans. 

Soils. — While  the  soils  of  alluvial  fans  like  those  of  deltas  are  trans- 
ported by  water,  yet  there  are  many  contrasts.  The  soils  of  fans  and 
cones  are  relatively  coarse,  typically  of  sands  and  gravels,  because 
first  they  have  been  transported  but  a  comparatively  short  distance 
and  secondly  in  the  characteristic  dry  regions  the  rocks  are  broken  down 

mainly  by  disintegration  and  this 
process  typically  yields  coarse 
materials.  Then  naturally,  it 
follows  that  in  the  dry  regions 
these  soils  are  comparatively 
unleached  and  ordinarily  contain 
abundant  mineral  plant  food,  but, 
however, .the  amounts  of  plant 
food  will  obviously  depend  on 
the  parent  rocks.  Since  the 
streams  building  fans  are  com- 
paratively short  and  their  basins 
small,  the  materials  in  the  soils 
show  a  close  relationship  to  the 
original  rocks,  a  much  closer 
relationship  than  is  usually  found 
in  delta  soils. 

In  general,  the  soils  of  alluvial 
fans  are  gravelly  or  sandy,  typi- 
cally coarse  gravelly  sands  in  the 
upper  and  higher  portions  grading 
to  sands  and  sandy  loams  in  the 
lower  portions.     The  distributar- 
ies or  "washes  "  are  commonly  dry  and  marked  by  long  tongues  of 
coarse  gravel.     The  distribution  of  soils  is  naturally  somewhat  irregular, 


FIG.  154. — Soils  on  a  portion  of  a  pied- 
mont alluvial  fan  at  the  base  of  the 
Sierra  Nevada  Mountains,  Cal.  The 
numbers  with  the  profile  on  the  right 
show  the  altitudes.  (U.  S.  Bureau  of 
Soils.) 


ALLUVIAL  FANS  AND  CONES 


177 


owing  to  variations  in  the  volume  and  load  of  the  feeding  streams. 
The  map,  Fig.  154,  shows  the  soils  on  a  portion  of  a  compound  alluvial 
fan  in  the  Valley  of  California.  In  this  case  the  coarse  gravelly  materials 
cover  most  of  the  fan  while  the  sand  and  sandy  loams  are  on  the  mar- 


FIG.  155. — Alluvial  fans  extending  from  the  base  of  the  Coast  Range,  Cal.     The 
fans  are  much  eroded.     (Prof.  C.  F.  Shaw,  Univ.  of  Cal.) 

ginal  portions.  It  must  be  remembered,  however,  that  the  relative 
proportions  of  coarse-  and  fine-textured  soils  vary  greatly  in  different 
fans. 

REFERENCES— Streams  and  Stream  Work 

CHAMBERLIN  and  SALISBURY,  Geology,  Holt,  Vol.  1,  Chapter  3. 

A.  W.  GRABEAU,  The  Principles  of  Stratigraphy,  Seiler,  1913:  River  Currents,  pages 
244-257  (abrasion). 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  Chap- 
ter on  Alluvial  Deposits. 

I.  C.  RUSSELL,  Rivers  of  North  America,  Putnam,  New  York,  1898. 

R.  D.  SALISBURY,  Physiography,  Holt,  1907,  Chapter  4. 

TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapter  5-6. 

THOMAS  and  WATT,  Improvement  of  Rivers,  2  vols.,  2d  Edition,  Wiley  &  Sons, 
1913. 

Soil  Erosion 

L.  C.  GLENN,  Denudation  and  Erosion  in  the  Southern  Appalachian  and  the  Monon- 
gahela  Basin,  Professional  Paper  72,  U.  S.  Geological  Survey,  1911,  pages  2-25 
(general  discussion). 

W.  J.  McGEE,  Soil  Erosion,  Bull.  71,  U.  S.  Bureau  of  Soils,  1911. 


178  CLASSES  OF  ALLUVIAL  DEPOSITS 

Alluvial  Fans 

W.  D.  JOHNSON,  The  High  Plains  and  Their  Utilization,  21st  Ann.  Kept.,  Part  4, 
U.  S.  Geological  Survey,  pages  612-657. 

Alluvial  Soils 

H.  H.  BENNETT,  Soils  of  the  River  Flood  Plain  Province  in  Soils  of  the  United  States, 
Bull.  96,  U.  S.  Bureau  of  Soils,  1913;  General,  pages  303-310;  Soil  Series, 
pages  310-380. 

C.  F.  MARBUT  and  J.  E.  LAPHAM,  Soils  of  the  Glacial  Lake  and  River  Terrace  Province, 
Bull.  96,  U.  S.  Bureau  of  Soils,  1913,  pages  165-220. 


THE  CYCLE  OF  EROSION 

It  is  at  once  evident  that  the  topography  produced  by  erosion 
will  change  as  erosion  goes  on.  There  is  considerable  aid  in  under- 
standing and  interpreting  land  surfaces  if  they  are  divided  into  three 
stages,  youthful,  mature  and  old;  youthful,  when  most  of  the  work  of  a 
stream  system  is  yet  to  be  done,  mature  when  stream  erosion  is  at  its 
maximum,  and  old  when  erosion  has  nearly  finished  its  work.  The 
time  required  for  a  land  surface  to  pass  through  these  three  stages  is 
termed  the  cycle  of  erosion.  It  should  be  remembered  that  youth, 
maturity  and  age  are  stages,  not  ages.  The  oak,  for  example,  is  young 
at  seven  months,  while  corn  is  old  at  that  age.  Similarly,  a  region  under- 
lain by  weak  rocks  may  be  worn  to  late  maturity  or  age  while  an  adja- 
cent region  underlain  by  strong  rocks  may  still  be  in  youth.  For 
example,  the  upper  Osage  valley  in  Missouri,  Fig.  156,  is  underlain 
mainly  by  weak  shales  and  is  worn  to  a  rolling  surface  nearly  in  the 
stage  of  age  while  the  lower  valley  which  is  really  older  in  age  is  under- 
lain by  resistant  sandstone,  the  surface  of  which  is  in  rugged  maturity. 

The  cycle  of  erosion  can,  perhaps,  be  best  explained  by  a  somewhat 
ideal  case  as  follows,  Fig.  157. 

Youth. — Let  us  assume  that  a  fresh  uneroded  surface  underlain,  say, 
by  limestone  is  exposed  to  erosion.  At  first  the  streams  are  not  numer- 
ous and  there  are  but  few  tributaries.  Streams  flow  in  narrow,  steep- 
sided  valleys  because  there  has  not  yet  been  time  for  deep,  wide  valleys 
to  be  developed.  Divides  are  flat  and  often  poorly  drained  with, 
perhaps,  lakes  and  swamps.  .  The  run-off  is  low  and  much  of  the  rair.- 
fall  sinks  into  the  ground  or  stands  for  a  long  time  on  the  surface.  An 
excellent  example  of  youthful  topography  is  seen  in  the  valley  of  the 
Red  River  of  the  North.  Formerly  the  site  of  a  lake,  the  smooth  lake 
bottom  has  been  eroded  but  a  short  time  as  time  is  estimated  in  geology. 


THE  CYCLE  OF  EROSION 


179 


Maturity. — As  erosion  goes  on  the  topography  changes.  Stream 
tributaries  increase  in  number,  valleys  are  worn  wider  and  deeper  and 
slopes  become  steeper.  Divides  become  narrower  and  sharper.  Rain- 
fall finds  its  way  quickly  into  the  streams  and  erosion  is  at  its  maximum. 
Lakes  and  swamps  have  been  filled  or  drained.  Late  in  this  stage  the 
lower  portions  of  streams  have  become  graded,  they  swing  from  side  to 


FIG.  156. — The  upper  Osage  valley  is  underlain  by  weak  rocks  (W)  and  is  eroded 
to  an  old  surface.  The  lower  valley  underlain  by  strong  rocks  ($)  is  eroded 
only  to  a  mature  stage.  The  river  flows  from  left  to  right. 


FIG.  157.— Diagram  illustrating  the  cycle  of  erosion.     The  area  in  youth  at  the  left 
changes  to  maturity  and  later  to  age. 

side,  thus  widening  their  valleys,  and  narrow  strips  of  flood  plain  may  be 
laid  down.  Such  an  area  is  often  described  as  mountainous.  Many 
areas  of  the  Allegheny  and  Cumberland  Plateaus  are  in  this  stage. 

Age. — With  the  passing  of  maturity,  the  larger  streams  begin  to 
lose  their  vigor  and  deposit  rather  than  cut,  except  as  they  may  swing 
from  side  to  side  and  cut  laterally.  The  valleys  become  shallower,  both 
from  filling  and  from  the  wearing  down  of  the  divides,  which  assume 


180  CLASSES  OF  ALLUVIAL  DEPOSITS 

low,  rounded  contours.  Stream  erosion,  so  effective  in  the  preceding 
stages,  is  largely  replaced  by  weathering  and  solution.  The  sluggish 
streams  meander  in  wide  valleys.  In  short,  the  agents  of  erosion  are 
working  slowly  and  the  land  surface  is  very  slowly  reduced.  Where 
this  stage  is  prolonged,  so  that  the  land  is  worn  to  a  low  featureless  plain 
but  little  above  sea  level,  such  a  plain  is  called  a  peneplain. 

There  are  considerable  areas  in  eastern  Kansas  and  western  Mis- 
souri which  have  been  eroded  to  an  old  stage  because  the  underlying 

rocks  are  easily  eroded,  Fig. 
158.     Indeed  so  long  a  period 
is  required  to  reduce  a  sur- 
face  to   age   that    no   large 
typical    area    in   this    stage 
has    been   found.      In    past 
ages,     however,     peneplains 
FIG.  158. — The  :_ocks  (shales)  are  weak  and      have    been   formed    and    re- 
have  been  worn  to  a  stage  of  early  age,      elevated  and  their  old   level 
Southwestern  Missouri.  gurfaces     are     preserved     Qn 

hill  and  ridge  tops.     Such  a 

peneplain  is  indicated  in  Fig.  58,  where  the  old  level  surface  is  preserved 
on  the  ridge  tops. 

Stages  and  Soils. — Agricultural  conditions  and  soils  show  close 
relations  to  the  stages  in  the  cycle  of  erosion.  In  youth  the  run-off  is 
slight,  much  water  soaks  into  the  ground  and  leaching  is  at  a  maximum. 
Typically  the  drainage  is  poor.  The  surface  is  level  and  well  adapted 
to  the  use  of  labor-saving  machinery.  In  maturity,  slopes  are  steep, 
soil  washing  and  erosion  are  at  the  maximum  and  soils  are  thin  and 
stony.  Arable  land  is  at  the  minimum  and  the  regions  are  typically 
adapted  to  grazing  and  forestry.  As  the  stage  of  age  is  reached,  the 
slopes  again  become  gentle,  the  topography  loses  the  ruggedness  of 
maturity  and  becomes  rolling.  As  in  youth  solution  and  leaching 
become  prominent  and  a  deep  mantle  of  residual  soil  covers  the  uplands, 
a  mantle  which  has  been  thoroughly  weathered.  Alluvial  soils  are  of 
considerable  extent  and  the  percentage  of  arable  soils  is  high.  While 
there  are  no  large  areas  that  are  in  typical  age,  there  are  many  small 
areas  that  are  worn  down  to  gently  rolling  surfaces. 

REFERENCES 

W.  M.  DAVIS,  Geographical  Essays,  Ginn,  Chapter  13. 

TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapter  7. 


CHAPTER  IX 
SOIL  CREEP.     COLLUVIAL  SOILS 

Introductory. — It  is  evident,  even  to  the  casual  observer,  that  fine 
soil  and  rock  particles  are  being  washed  from  the  lands  into  streams  and 
carried  into  oceans  by  the  agency  of  running  water.  The  much  slower 
movement,  by  which  the  waste  from  the  lands  is  as  surely  moving  to 
the  oceans,  is  usually  overlooked.  Occasionally  a  boulder  on  a  steep 
slope  can  be  observed  to  have  moved  down  slope  a  few  inches  during  a 
generation;  of  course,  the  mantle  rock  adjacent  to  the  boulder  has  also 
moved,  but  its  progress  is  not  so  readily  noted.  Again,  to  use  another 
illustration,  the  boulder  and  its  adjacent  mantle  rock  may  slide  down 
slope  as  an  avalanche.  These  two  movements,  the  one  very  slow  and 
the  other  rapid,  are  mainly  due  to  gravity  without  the  aid  of  running 
water.  Except  where  the  surface  is  level,  there  is  this  slow  movement 
of  mantle  rock  the  world  over.  Both  the  slow  creep  and  the  relatively 
rapid  landslides  are^  sources  of  trouble  in  railroad  construction  and 
maintenance,  since  the  movements  displace  the  tracks.  The  landslides 
along  the  Panama  Canal  have  become  famous.  There  is  no  widely 
accepted  term  to  apply  to  this  movement  as  a  whole,  by  which  land 
waste  is  moved  down  slope  without  the  aid  of  streams;  it  may  be  con- 
sidered under  three  heads:  (1)  soil  creep  or  solifluction,  (2)  talus  accumu- 
lations and  (3)  landslides.  Soils  due  to  these  processes  are  termed 
colluvial  soils. 

Soil  creep,  as  the  name  implies,  is  the  very  slow  movement  of 
land  waste  down  slopes,  "  a  slow  washing  or  creeping  of  the  waste  down 
the  land  slopes;  not  bodily  or  hastily  but  grain  by  grain,  inch  by  inch; 
yet  so  patiently  that,  in  the  course  of  ages,  even  mountains  may  be  laid 
low  "  (Davis).  It  is  usually  imperceptible  from  generation  to  genera- 
tion, although  leaning  trees  often  indicate  this  movement. 

Factors. — (1)  The  primary  agent  of  soil  creep  is  gravity  and  the 
primary  factor  is  slope.  Other  things  being  equal,  soil  creep,  like  the 
analogous  flow  of  streams,  is  the  faster  the  steeper  the  slope.  (2)  If  the 
soil  mantle  is  saturated  with  water,  friction  between  the  particles  is 

181 


182 


SOIL  CREEP.     COLLUVIAL  SOILS 


lessened  and  the  creep  is  accelerated;  there  is  no  sharp  distinction 
between  water  filled  with  land  waste  and  land  waste  saturated  with 
water,  each  moving  down  slope.  (3)  When  water  freezes  and  expands 

in  the    pore    spaces    of    soils 


particles    are    slightly   lifted 
and,  as  thawing  follows,   the 
FIG.  159.-Diagram  to  illustrate  the  move-      Particles  sink  back  vertically 
ments  of  soil  particles  due  to  freezing  and      on    level    areas    but    diagon- 
thawing  on  level  and  on  steep  slopes.  ally  on  slopes.     This  is  illus- 

trated in  Fig.    159,  where  it 

is  seen  that  the  successive  frost  heavings  result  in  a  very  slow 
migration  of  particles  down  slope.  Thus  the  whole  mass  of  loose 
rock,  gravel,  sand,  clay  and  soil  moves  down  hills  and  other  slopes, 
pausing  here  and  there  behind  obstructions  or  where  the  slopes  become 


mm  vw 
RiflM;, 


FIG.  160. — Whiteside  Mountain,  Southern  Appalachians.     Soil  lodging  and  accumu- 
lating on  more  gentle  slopes.     (Glenn,  U.  S.  Geological  Survey.) 

more  gentle,  Fig.  160.  (4)  Finally,  the  loose  materials  come  to  rest  at 
the  foot  of  a  slope  and  begin  to  accumulate,  provided  they  are  not 
washed  away.  As  the  detritus  accumulates  the  slopes  become  more 
gentle  and  so  facilitate  further  deposition  until  a  low  bulging  or  "  shoul- 


ASSOCIATED  AGENTS 


183 


der  "  of  colluvial  materials  may  be  built,  Fig.  161.  Some  kinds  of  arti- 
ficial terraces  are  completed  in  part  by  soil  creep;  when  a  retaining 
wall  is  built  in  some  regions  of  rapid  soil  creep,  the  space  on  the  up- 
hill side  of  the  wall  will  be  filled  to  some  extent  with  colluvial  soil. 


FIG.  161. — :A  "shoulder"  of  colluvial  soil  at  the  base  of  a  sandstone  hill, 
buildings  are  on  the  colluvial  soil,  Kansas. 


The 


Associated  Agents. — Like  most  other  geological  processes,  soil  creep 
seldom  acts  alone,  but  cooperates  with  other  agents.  For  instance,  it  is 
nearly  always  accompanied  by  sheet  wash,  that  is,  the  flow  of  water 
over  the  surface  as  a  sheet  instead  of  as  a  stream.  This  can  easily 

be  observed  on  a  small  area 
during  a  heavy  rain;  the 
areas  affected  are  small,  but 
there  are  many  of  them,  and 
they  f  are  largely  confined  to 
the  uppermost  portions  of 
the  mantle  rock,  Fig.  .162. 
Closely  allied  with  the  sheet 
wash  is  the  slower  movement 
down  slope  of  the  upper 
ground  water  which  carries 
fine  silts  and  clays  for  short 
distances. 

Important  areas  of  soils 
affected  by  these  movements 
are  found  around  the  heads 

of  streams  where  head  erosion  (page  137)  is  active  and  the  steeper 
slopes  here  favor  active  soil  creep.  Another  important  cooperation 
of  agents  producing  soil  creep  is  seen  on  long,  gentle  slopes.  The 
slow  journey  of  the  rock  fragments  allows  the  processes  of  weather- 


FIG.  162.— Sheet  wash,  Alaska.    (Moffit,  U.  S. 
Geological  Survey.) 


184 


SOIL  CREEP.    COLLUVIAL  SOILS 


ing  to  act  on  them  for  a  long  time  and  the  rocks  become  progressively 
smaller  as  they  move  down  slope.     As  a  consequence  there  is  often  a 

fairly  regular  change  from  stony 
ground  on  upper  slopes  through 
stony  loams,  gravelly  loams, 
sandy  loams  to  fine-textured 
loams  on  lower  slopes. 

Differential  Movements. — 
It  should  further  be  noted  that 
not  all  loose  materials  move 

down  slope  at  the  same  rate. 

riG.  163. — Contour  terraces  to  hold  colluvial      Tr   i         .1         i 
soil  and  to  prevent  soil  erosion,  N.  C.  (U.S.      Unless  the    sl°Pes    are  SO  steep 
Bureau  of  Soils.)  tnat  tneY  ro11  down,  the   rocks 

and  boulders  move  much  slower 

than  the  fine  materials  and,  as  a  result  of  this  differential  movement, 

the   rocks    and   boulders  lag  behind  the  sand,  silt    and  clay  so  that 

the  upper  slopes  are 

stony  while  the  lower  ""9     ^- 

slopes    are    covered 

with      fine  -  grained 

colluvial    materials. 

A    typical    illustra- 
tion   is    shown     in 

Fig.  164  which  shows 

soil  sections  of  limestone  hills,  one  flat  topped  and  the  other  curved 

topped.     The  steep  slopes  are  stony  because  much  of  the  finer  material 

has  moved  down  to  the  belt 
where  colluvial  soils  are  accu- 
mulating. The  level  surface 
of  the  flat-topped  hill  favors 
the  formation  of  a  residual  soil, 
but  in  time  its  summit,  like  the 
other,  will  be  eroded  to  a  curved 
surface  when  its  soils  will  be- 
come stony.  In  resume,  it  may 
be  said  that  in  regions  of  col- 


FIG.  164. — Diagram  to  illustrate  the  occurrence  of  resid- 
ual and  coliuvial  limestone  soils  on  (A)  a  flat-topped 
hill  and  (B)  a  round-topped  hill.  (1)  colluvial  loams; 
(2)  stony  loams;  (3)  residual  silt  loams. 


FIG.  165. — Diagram  to  illustrate  the  effects 
of  soil  creep  and  head  erosion  on  lime- 
stone soils,  Kansas:  (1)  stony  loam;  (2) 
shallow  stony  loam;  (3)  silt  loam.  (Data 
from  U.  S.  Bureau  of  Soils.) 


luvial  soils  the  upper   zone   is 
typically    of    coarse    textured, 

often  stony  soils,  where  erosion  and  soil  creep  is  predominant  and  the 
lower,  narrower  zone  is  composed  of  fine-textured  soils  where  accu- 


SOIL  CREEP  AND   ROCK  VARIATION  185 

mulation  is  predominant.  In  other  words,  erosion,  including  soil 
creep,  sheet  wash  and  the  head  erosion  of  streams  exceeds  weathering 
on  upper  slopes  while  weathering  and  accumulation  predominate  on 
lower  slopes.  Some  of  these  effects  are  shown  in  Fig.  165. 

Soil  Creep  and  Rock  Variation 

Not  all  colluvial  materials  are  so  simple  in  origin.  A  complicating 
factor  is  the  variety  of  rock  from  which  the  soils  are  derivecj,  as  is 
indicated  in  Fig.  166.  In  one  case,  Fig.  166^1,  a  phosphatic  limestone 
(LS)  is  underlain  by  shale. 

The  fine    sands    and    clays,        jlSSl^s^  j:J^^^^   B 

together  with  soluble  phos- 
phates and  bits  of  the  lime- 
stone,  will    move    down  the      FIG.  166.— Diagram  to  illustrate  origin  of  col- 
slope,  mingle    with    the  silts          luvial  materials.     (LS)  phosphatic  limestone; 
and    clays    from    the    shale         (SH>  shale'  (ss>  sandstone. 
(SH)  and  accumulate   over 

the  less  productive  sandstone  (SS),  forming  a  belt  of  fertile  colluvial 
soil.  On  the  other  hand,  if  the  less  productive  sandstone  is  at  the 
top  of  the  slope,  the  sand  and  sandstone  rocks,  together  with  the 
materials  from  the  shale,  will  cover  the  productive  limestone  soils  with 
a  much  less  fertile  soil  belt.  These  will  illustrate  two  of  the  many 
rock  variations  which  complicate  the  distribution  of  colluvial  soils. 

In  general,  it  may  be  said  that  in  sandstone  regions  with  colluvial 
soils,  the  upper  slopes  are  sandy  if  the  sandstone  is  weakly  cemented 
and  stony  if  the  underlying  sandstone  is  resistant;  on  the  lower  slopes 
the  colluvial  soils  are  composed  of  the  very  fine  sand  and  silt  that  has 
moved  down  slope.  In  limestone  regions  the  upper  belt  is  very  likely 
to  be  strewn  with  cherty  fragments  since  most  limestones  contain 
some  of  this  resistant  material.  The  lower  colluvial  belt  of  limestone 
soils  is  often  a  silt  loam  containing  fine  chert  grains.  The  soils  from 
shales  in  hilly  regions  are  variable  but,  in  general,  the  upper  slopes  are 
often  "  slaty  "  or  largely  of  shale  loam  and  the  colluvial  soil  belt  is  often 
silt  or  clay  loam  with  which  are  intermingled  flat  fragments  of  shale. 
The  colluvial  soils  from  granites,  gneisses  and  schists  are  extremely 
variable  and  about  all  that  can  be  said  in  this  connection  is  that  there 
are  usually  the  two  typical  upper  and  lower  zones  of  relatively  coarse 
and  fine  materials.  In  limestones  and  sandstones  especially  and  in 
many  other  rocks  as  well,  a  soil  map  gives  a  fairly  accurate  topographic 


186  SOIL  CREEP.     COLLUVIAL  SOILS 

map  so  far  as  the  slopes  are  concerned,  the  stony  soils  mapping  the  upper 
slopes  and  the  colluvial  the  lower  slopes. 

Not  only  are  soil  creep  and  the  commonly  associated  soil  erosion 
found  in  regions  of  hard  rock,  but  these  processes  are  pronounced  in 
many  regions  of  unconsolidated  rocks.  Some  of  these  formations  are 
relatively  young  and  soil  creep  has  not  been  in  operation  long  enough 
to  produce  well-marked  colluvial  soils  and  some  of  the  young  forma- 
tions like  the  sandy  Lafayette  Formation  in  the  South  are  so  porous 
that  the  rains  sink  without  much  surface  wash  or  creep.  Even  in  these 
formations,  however,  the  hill  tops  are  very  often  more  sandy  than  the 


FIG.  167. — Both  soils  are  derived  from  sandy  shales.  In  the  region  on  the  left, 
the  slopes  are  so  steep  that  the  soils  are  stony  loams.  The  more  gentle  slopes  on 
the  right  have  caused  the  formation  of  silt  loams,  W.  Va.  (U.  S.  Bureau  of 

Soils.) 

lower  hill  slopes.  Soil  creep,  together  with  some  soil  erosion,  b  seen  in 
hilly  regions  of  glacial  materials  where  drumlins,  which  are  hills  com- 
posed mostly  of  clay,  Fig.  191,  often  have  slightly  eroded  tops,  locally 
called  "  balds." 


Colluvial  Soils 

While  theoretically  the  soils  on  all  slopes  are  subject  to  soil  creep, 
yet  the  term  colluvial  soils  is  restricted  to  belts  of  soils  which  have  crept 
down  slopes  and  accumulated  in  belts  wide  enough  to  have  agricultural 
interest.  They  are  most  important  in  hilly  and  humid  regions,  although 
they  are  often  found  in  arid  regions  as  well.  The  aggregate  areas  of 
soils  affected  by  soil  creep  is  very  large  and  includes  nearly  all  habitable 
areas  where  there  are  well-marked  slopes.  In  regions  like  the  lime- 


TALUS  187 

stone  Ozarks  of  Missouri  the  narrow  belts  of  colluvial  soils  are  in  many 
places  the  only  available  soils  and  roads  are  run  on  the  adjacent  stony 
areas  so  as  to  save  the  colluvia!  soils  for  cultivation.  Here,  as  in 
many  other  places,  the  upper  stony  areas  are  usually  left  in  timber  and 
brush,  while  the  lower  slopes  are  cleared,  and  many  farms  here  include 
large  stony  areas  for  pasture  and  much  smaller  areas  of  colluvial  soils 
for  crops. 

The  famous  "  black  pippin  soil  "  in  the  coves  of  the  Blue  Ridge  of  Virginia  is 
largely  colluvial.  These  coves  are  made  by  head  erosion  as  streams  push  their 
headwaters  back  into  the  crystalline  rocks.  The  loose  materials  are  washed  into 
or  creep  into  these  coves  faster  than  the  small  streams  can  remove  them  so  that 
a  deep,  rich,  black  soil  has  accumulated.  This  soil,  together  with  the  protection 
from  winds  that  is  afforded  by  the  cove  walls,  has  made  these  coves  very  favorable 
locations  for  orchards. 

Talus 

Talus  is  an  accumulation  of  materials,  mostly  coarse,  at  the  foot 
of  steep  slopes.  These  accumulations  belong  in  the  same  class,  so  far 
as  origin  is  concerned,  as  colluvial  soils  into  which  their  materials  often 
grade  and  with  which  their  materials  are  .often  intermingled.  The 
rate  and  kind  of  talus  accumulations  are  influenced  by  three  factors, 
(1)  steepness  of  slopes,  (2)  resistance  of  the  rocks,  and  (3)  climate. 

(1)  Steepness  of  slope. — In  regions  of  steep  slopes  the  coarse  mate- 
rials move  so  rapidly  that  they  are  not  much  weathered  in  transit  and 
they  arrive  at  the  slope  base  in  large,  angular  fragments.  When 
slopes  are  more  gentle,  the  fragments  undergo  a  slower  journey,  they 
are  longer  subject  to  weathering  and  the  fragments  are  smaller.  It 
has  been  noted  before  that  long,  gentle  slopes  favor  finer  colluvial 
materials  more  than  short  slopes.  (2)  Resistance  of  rock  is  an  obvious 
factor.  Other  things  being  equal,  the  talus  materials  from  quartzite 
ledges,  for  instance,  will  be  coarser  than,  those  from  limestone  ledges. 
In  this  connection,  it  must  be  remembered  that  mountainous  and  hilly 
regions  are  usually  underlain  by  resistant  rocks  so  that  talus  materials 
are  characteristically  coarse.  (3)  Rapidity  of  weathering.  The  more 
active  the  weathering  processes,  the  finer  will  be  the  colluvial  materials 
when  they  arrive  at  the  bases  of  slopes.  A  humid  climate  is  less  favor- 
able for  talus  accumulations,  for  the  materials  are  finer  and  there  is 
more  running  water  to  wash  them  away.  The  most  notable  talus  accu- 
mulations are,  therefore,  found  in  arid  and  semi-arid  regions.  In  some 
parts  of  the  Great  Basin  mountains  are  partly  buried  in  their  own  talus 
materials. 


188 


SOIL  CREEP.    COLLUVIAL  SOILS 


FIG.  168. — A  sandstone  cliff  with  talus 
extending  nearly  to  the  cliff  top. 
(Howe,  U.  S.  Geological  Survey.) 

weathering  of  talus  fragments,  or 
by  the  soil  creep  from  above.  Such 
soils  are  in  use  in  Switzerland  for 
pasture  and  for  orchards.  Talus 
soils  are  usually  thin,  coarse-text- 
ured and  droughty  and  will  not 
be  much  utilized  until  other  avail- 
able soils  are  brought  under  culti- 
vation; many  of  them,  "however, 
are  valuable  for  forestry 

Landslides  and  Avalanches 

These  occur  when  a  mass  of 
rock,  soils,  and  all  sorts  of  debris 
slide  down  slopes  in  areas  of .  a 
few  square  rods  to  areas  of  acres 
in  extent. 

is  the  principal  factor,  but  gravity 
is  aided  by  the  steepness  of  slopes 


Talus  is  found  at  the  base  of 
nearly  all  steep  rocky  slopes  and 
naturally  it  most  characteristi- 
cally occurs  where  these  accu- 
mulations often  fill  the  valleys 
between  mountains  to  a  consid- 
erable depth,  provided  there  is  no 
stream  able  to  carry  away  the 
fragments.  As  the  talus  accu- 
mulates it  extends  up  the  moun- 
tain side  in  a  curving  slope  and 
indeed  "  some  of  the  most  pleas- 
ing curves  in  mountain  topog- 
raphy are  those  of  the  talus 
slope  "  (Tarr  and  Martin).  Not 
infrequently  the  talus  becomes 
covered  with  vegetation,  either 
through  the  formation  of  soil  by 


TT  .  .,      FIG.    169. — Landslide   in    the    foreground. 

Here,  again,   gravity      Note  the  scar  on  the  mountain  from  which 


the  landslide  slipped,  Colo. 
Geological  Survey.) 


(Howe,  U.  S, 


LANDSLIDES  AND  AVALANCHES  189 

or  by  thorough  soaking  of  the  mantle  rock  and  often  by  smooth 
underlying  materials  like  clay,  which  allows  ready  sliding  of  the 
mass.  Fig.  169  shows  a  landslide  and  the  scar  where  the  mass  broke 
away.  Landslides  are  fortunately  rather  rare  except  in  a  few  regions 
of  steep  slopes  and  deep  mantle  rock,  but  the  movement  on  a  small  scale 
is  common.  Many  hillsides,  especially  pastures,  show  small  landslides 
of  a  few  feet,  often  one  above  the  other  like  rude  stairs.  These  small, 
usually  unnoticed  avalanches,  which  move  but  a  few  feet  or  a  few  inches 
at  a  time,  are  of  no  small  importance  in  the  general  down  slope  travel  of 
the  land  waste.  The  importance  of  soil  creep  combined  with  head 
erosion  is  often  overlooked  in  estimating  the  importance  of  soil  factors. 
Millions  of  acres  lying  on  slopes  have  their  agricultural  values  largely 
determined  by  these  processes. 

REFERENCES 

J.  G.  ANDERSSON,  Solifluction,  a  Component  of  Subaerial  Denudation,  Journal  of 
Geology,  Vol.  14,  1906. 

A.  W.  GRABOW,  The  Principles  of  Stratigraphy,  Seiler,  1913:  Slow  Movements  of 
Rock  and  Soil,  pages  543-548. 

WILLIAM  H.  HOBBS,  Soil  Stripes  in  Cold  Humid  Regions  and  a  Kindred  Phenomenon, 
12th  Report,  Mich.  Acad.  Sci.,  1910. 

EARNEST  HOWE,  Landslides  in  the  San  Juan  Mountains,  Colorado,  Professional 
Paper,  No.  67,  U.  S.  Geol.  Survey,  1909. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  Chap- 
ter on  Colluvial  Deposits. 

RIES  and  WATSON,  Engineering  Geology,  Wiley  &  Sons,  1914,  Chapter  7,  Land- 
slides and  Their  Effects. 

N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Report,  Part  1,  U.  S.  Geological 
Survey,  Cliff  Talus  (Colluvial  Soils),  pages  232-36. 


CHAPTER  X 
GLACIERS    AND    GLACIATION;     GLACIAL    SOILS 

Introductory. — The  study  of    glaciers    and  their  work  would  be 
interesting,  if  only  because  of  the  existing  glaciers,  which  are  visited  by 


FIG.  170. — Map  showing  the  glaciated  portions  of  North  America  and  the  centers 
from  which  the  glaciers  advanced.     (U.  S.  Geological  Survey.) 

190 


KINDS  OF  GLACIERS 


191 


thousands  of  tourists.  However,  when  it  is  remembered  that  consider- 
able areas  of  North  America  and  Europe  were  covered  by  great  glaciers 
during  the  Glacial  Period,  a  time  very  recent,  geologically  speaking,  the 
study  of  glaciers  acquires 
added  interest  and  value  (Figs. 
170  and  171).  During  this 
period  glaciers  of  enormous 
dimensions  spread  from  at 
least  three  centers  in  Canada 
and  extended  over  the  north- 
ern half  of  North  America. 
At  the  same  time  northwest- 
ern Europe  was  invaded  by 

the  ice.     Millions  of  people  in __ 

the  most  productive  parts  of     FlG  171._Map  showing  theparts  of  Europe 

the    earth    have    their     daily         affected  by  continental  glaciation  (dotted 

lives   influenced    by  the  work         areas).     (After  J.  Geikie.) 

of  extinct   glaciers.     It  is  by 

the  study  of  existing  glaciers  and  their  work  that  we  can  understand 

the  effects  of  glaciers  which  have  disappeared. 


KINDS  OF  GLACIERS 

For  convenience  of  study,  glaciers  are  divided  into  two  types,  moun- 
tain glaciers  and  continental  glaciers,  terms  which  are  self  explanatory, 
although  the  types  more  or  less  merge  into  each  other  and  have  many 
features  in  common 

Mountain  Glaciers 

When  the  snow  of  one  winter  does  not  entirely  melt  during  the 
summer  but  is  added  to  that  of  the  following  winter,  there  is  a  gradual 
accumulation  of  snow  which  may  result  in  a  glacier.  The  lower  layers 
are  compressed  into  ice  by  the  weight  of  the  overlying  snow  and  the 
mass  in  time  begins  to  spread,  and  this  spreading  movement  tends  to 
follow  the  valleys  and  ravines  which  lead  down  the  mountain.  Thus 
the  glaciers  move  downward  until  they  reach  such  a  distance  that  the 
rate  of  melting  equals  or  exceeds  the  rate  of  ice  advance.  The  distance 
which  a  glacier  may  move  in  a  year  or  a  series  of  years  depends  on  sev- 
eral factors,  among  which  are  changes  in  the  rate  of  ice  movement,  the 
amount  of  ice  and  the  character  of  the  seasons.  The  margin  of  the 


192  GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 

glacier,  therefore,  will  often  change  position:  it  is  a  common  experience 
in  the  Alps  that  a  cabin  built  near  a  glacier  may  be  destroyed  by  later 
advances^  of  the  ice.  On  the  northern  slopes  of  the  Alps,  the  glaciers 
descend  much  lower  than  on  the  southern  side  because  it  is  colder  on 
the  northern  side. 

In  following  up  a  mountain  glacier  one  first  crosses  the  rugged  and 
crevassed  ice  of  the  glacier  proper,  which  finally  leads  to  the  feeding 
snow  field  above  which  provides  the  material  for  the  ice.  Between  the 
snow  field  and  the  ice  is  usually  an  intermediate  zone  of  compacted 
and  granular  snow  called  the  neve. 

,-    •  '••'>•  •'£*.""•*"- 

Continental  Glaciers, 

as  the  name  implies,  are  of  great  extent  and  move  over  ridges,  hills  and 
valleys  with  but  little  regard  to  topography.  Greenland  and  Antarctic 
are  at  present  covered  with  glaciers  of  this  type;  in  the  Glacial  Period, 
this  type  was  the  prevalent  one.  It  is  obvious  that,  because  of  their 
inaccessibility,  extent  and  thickness,  continental  glaciers  are  difficult 
to  study  and  have  not  received  such  extended  and  careful  study  as  have 
mountain  glaciers.  Much  of  our  knowledge  of  glaciers  has  been 
gained  by  study  of  mountain  glaciers,  but  the  enormous  areas  formerly 
covered  by  vanished  glaciers  furnish  much  information  as  to  their  work. 

Conditions  of  Formation 

Two  conditions  are  necessary  for  the  formation  and  growth  of  glaciers, 
namely  (1)  sufficient  snow  fall  and  (2)  low  temperatures.  The  one  con- 
dition has  prevented  any  notable  growth  of  glaciers  in  many  parts  of 
the  frigid  zone  and  the  other  condition,  of  course,  excludes  glaciers  from 
hot  regions  except  on  high  mountains.  The  most  numerous  and  most 
familiar  glaciers  are  of  the  mountain  type  because  of  the  low  temperatures 
found  there.  Many  mountains  are  on  the  verge  of  supporting  glaciers 
since  a  winter's  snow  is  scarcely  melted  during  the  following  summer. 
It  will  be  clear  that  glaciers  and  a  glacial  period  do  not  necessarily  imply 
an  extremely  cold  climate. 

Movements 

The  exact  nature  of  ice  movement  is  not  well  understood,  but  it  is 
known  that  ice  behaves  somewhat  like  a  very  stiff  fluid.  It  has  been 


KINDS  OF  GLACIERS 


193 


FIG.  172.— Mt.  St.  Helens,  Alaska.  Glaciers 
are  descending  from  the  snowfields.  (U.  S. 
Geological  Survey.) 


noted  that  a  glacier  moves  down  a  valley,  a  constant  proof  of  which  is 
seen  when  the  baggage  of  mountain  climbers  is  lost  in  an  ice  crevasse 
and  years  later  reappears  at  the  foot  of  the  glacier;  another  evidence  of  a 
glacier's  movement  is  the 
progress  down  valley  of  large 
stones  on  the  ice  surface.  A 
descending  mountain  glacier 
flows  somewhat  like  a  stream; 
in  straight  reaches  the  fastest 
flow  is  usually  near  the  center 
and  in  bends  the  fastest  move- 
ment is  on  the  outer  side  of  the 
bend,  as  in  streams.  The  rate 
of  movement  is  ordinarily  very 
slow,  usually  only  a  few  inches 
a  day,  although  some  excep- 
tionally fast  glaciers  have  been 

known  to  move  over  fifty  feet  in  a  day.  Ice  movement  is  often  accom- 
panied by  much  crevassing  of  the  glaciers,  Fig.  172. 

The  rapidity  of  glacial  movement  varies  with  several  factors.     (1) 

Other  things  being  equal,  the  deeper  the  ice  the  faster  it  moves.     (2) 

The  ice,  like  water,  tends  to  move  fastest  over  steep  slopes.     (3)  Again, 

the  character  of  the  surface  over  which  the  ice  moves  has  some  influence; 

.  a  rough  uneven   surface   offers 

more  resistance  than  a  smooth 
surface.  (4)  Ice  flows  faster 
when  near  the  freezing  point; 
in  other  words,  the  higher  the 
temperature  of  the  ice;  provided 
it  does  not  rise  above  the  freez- 
ing point,  the  faster  the  ice 
tends  to  move.  (5)  Finally, 
FIG.  173.— Crevassed  surface  of  a  glacier,  when  ice  is  heavily  loaded  with 
Canada.  (Canadian  Geological  Survey.)  boulders,  sand  and  other  debris, 

it  tends  to  move  more  slowly. 

Some  of  these  factors  at  times  cooperate  so  as  to  cause  notable  advance 
of  a  glacier  and,  again,  they  act  in  such  a  fashion  as  to  cause  the 
glacier's  front  to  remain  practically  stationary. 

Ice  Advance  and  Retreat. — In  preceding  pages  the  advance  of  the 
ice  has  been  emphasized,  for  most  of  the  ice  erosion  is  accomplished 


194  GLACIERS  AND  GLACIATION:  GLACIAL  SOILS 

during  the  glacier's  advance.  It  will,  of  course,  be  understood  that  the 
retreat  of  a  glacier  is  simply  a  passive  melting  back  of  the  ice  front  while 
the  ice  advance  is  an  active  forward  movement.  Whether  the  front  of  a, 
glacier  advances  or  retreats  or  remains  practically  stationary  is  depend- 
ent on  the  relation  of  the  ice  advance  to  the  melting  at  the  ice  front. 
Naturally,  in  one  year  or  a  series  of  years,  if  the  advance  of  the  ice  is 
greater  than  the  rate  of  melting,  the  front  of  the  glacier  will  advance, 
but  if  the  melting  or  evaporation  of  the  ice  should  increase  so  that 
the  rate  of  ice  wastage  be  greater  than  the  rate  of  advance,  the  glacier's 
front  will  retreat.  Practically  all  glaciers  repeatedly  advance  and 
retreat  and  some  glaciers  advance  in  one  place  and  retreat  in  another. 
The  Kewaunee  soils  which  occur  in  the  regions  of  Lakes  Superior  and 
Michigan  have  been  developed  by  the  advances  of  glaciers  over  reddish 
silty  clays  which  have  been  mixed  both  in  soils  and  subsoils  with  gravel, 
stones  and  boulders  from  the  glacier. 

REFERENCES 

W.  H.  HOBBS,  Characteristics  of  Existing  Glaciers,  Macmillan,  1911    (especially 

pages  1-90). 
I.  C.  RUSSELL,  Glaciers  of  North  America,  Ginn,  1897:  Living  Glaciers,  pages  1-16; 

Abrasion,  pages  16-22;  Deposits,  pages  22-31. 
TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  pages  197-204. 

THE  WORK  OF  GLACIERS 

Introductory. — The  work  of  glaciers  is  both  constructive  and  destruc- 
tive. In  the  ice  advance  the  work  is  mainly  destructive  and,  in  the  ice 
retreat,  constructive  work  predominates.  In  regions  of  rugged  topog- 
raphy the  ice  may  make  but  little  change  in  either  the  major  or  the 
minor  features  but,  on  the  other  hand,  the  minor  preglacial  features 
may  be  nearly  obliterated,  the  drainage  deranged  and  the  general  aspect 
of  the  region  much  changed,  as  in  some  parts  of  the  upper  Mississippi 
Basin.  In  some  places  the  preglacial  soils  have  scarcely  been  changed, 
while  in  other  regions  the  soils  bear  little  relation  to  the  underlying 
rocks.  It  is  probably  safe  to  state  that,  from  the  viewpoint  of  soils, 
there  is  no  other  single  factor  so  important  in  North  America  as  gla- 
ciation  and  the  same  is  true  over  much  of  Northern  Europe. 

EROSION  AND  TRANSPORTATION  BY  GLACIERS 

Ice  Tools. — Pure  ice,  like  clear  water,  can  accomplish  comparatively 
little  erosion  but,  when  armed  with  boulders,  pebbles  and  sand  held 


THE  WORK  OF  GLACIERS 


195 


FIG.  174. — Unglaciated  hill  top  above,  Virginia.    Below,  glaciated  hill  top,  Conn. 
(Conn.  Geological  Survey.) 


196 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


in  the  lower  layers,  the  glacier  acquires  enormous  abrasive  power  and 
may  scour  to  depths  of  scores  of  feet.  Thus  armed,  the  glacier  "  may 

be  compared  to  a  huge,  flexible 
rasp,  fitted  down  snugly  over 
hills     and     depressions,    and 
urged    slowly   forward  under 
enormous  pressure."     (Cham- 
berlin.)     Thousands  of  expo- 
sures of  hard  rocks  show  the 
scratches  or    strice    made    by 
stones   embedded   in    the  ice 
and    dragged    over   the    rock 
FIG.  175.— Smoothed  and  grooved  glaciated      surfaces,  Fig.   175,  and  stones 
rock  surfaces.     (Atwood,  U.  S.  Geological      and  pebbles  are  often  ground 
Survey.)  off    when    thus   held    in    the 

ice,  Fig.  176,  and  many  small 

and  large  rocks  that  have  been  carried  in  the  ice  show  scratches. 
The  striae  on  bed  rocks,  besides  being  '  interesting  as  evidences 
of  glaciation,  have  much  scientific  value,  because  they  show  the 
general  direction  of  ice 
movement. 

The  vigor  of  ice  ero- 
sion varies  with  several 
factors.  (1)  The  faster 
the  ice  moves  the  greater 
the  erosion,  other  things 
being  equal.  (2)  It  is 
evident  that  the  thick- 
ness of  the  ice  is  an 
important  factor,  for  the 
thicker  the  ice  the 
greater  pressure  upon 
the  surface  beneath  the  FlG>  170.— Stone  scratched  and  smoothed  by 
glacier  and  the  more  glaciers, 

effective  is  the  scouring 

of  the  underlying  rock.  (3)  Again,  the  length  of  time  during  which 
a  glacier  moves  over  a  given  region  is  an  important  factor.  Two 
rock  factors  are  important.  (4)  The  resistance  of  rocks  varies; 
obviously  a  granite  will  offer  more  resistance  to  abrasion  than  a 
shale.  (5)  Then,  if  stratified  rocks  dip  away  from  the  direction  of 


THE  WORK  OF  GLACIERS  197 

ice  movement,  the  conditions  are  more  effective  for  prying  and  pluck- 
ing, as  shown   in   Fig.    177.      (6)    Finally,  as  in  water   erosion,    the 
character    of  the  tools    is   important;   hard,  angular  large  rocks  are 
especially    effective    when     em- 
bedded in   the  moving  ice  and 
pressed  down  upon  the  rock  sur- 
face beneath  the  ice. 


In  rugged  regions  like  New  FlG<  177.— Diagram  to  illustrate  plucking 
England,  the  ice  swept  away  by  ice  when  the  rocks  dip  away  from 
much  of  the  preglacial  soil  and  the  glacier's  movement.  The  arrows 
loose,  decayed  rock,  but  even  show  the  direction  of  the  ice  movement, 
here  the  glaciers  did  not  remove 

all  the  loose  materials  in  all  places.  On  the  other  hand,  in  less  rugged 
regions  like  the  plains  of  the  upper  Mississippi  Basin  the  glaciers  over- 
rode large  areas  of  soils  with  but  little  disturbance  of  them,  and  often 
buried  soils  are  found  in  wells,  soils  that  have  been  overlain  by  later 
glacial  deposits. 

Plucking. — However,  even  clear  ice  will  pry  off  or  pluck  projecting 
pieces  of  rock,  often  of  great  size,  and  bear  them  away.  This  smoothing 
by  plucking  is  shown  in  many  valley  sides.  A  deep  valley  produced 
by  stream  erosion  is  likely  to  have  rough  sides;  spurs  project  and  rock 
buttresses,  often  called  "  chimneys,"  pinnacles,  etc.,  are  often  to  be 
found.  These  forms  are  either  reduced  or  much  modified  when  a 
glacier  passes  through  a  valley,  plucks  off  the  projecting  rocks  and 
abrades  the  walls.  Sometimes  in  a  deep  valley  the  lower  part  of  the 
valley  has  been  smoothed  by  glaciation  while  the  upper  portions,  which 
the  glacier  did  not  reach,  retain  the  ruggedness  due  to  ordinary  erosion, 
Fig.  178.  Obviously,  glacial  soils  are  likely  to  be  bouldery  where  plucking 
was  prominent.  It  is  usual  to  find  bouldery  soils  where  glaciers  have 
passed  over  rock  ledges,  plucked  off  fragments  and  strewn  them  about. 

The  Ice  Lead  and  its  Transportation. — In  contrast  with  water  and 
wind,  glaciers  transport  all  fragments  irrespective  of  size.  For  the  most 
part  the  load  is  acquired  and  carried  in  the  lower  portions  of  glaciers, 
especially  in  continental  glaciers.  The  ice  undoubtedly  advanced 
very  slowly  with  but  little  violent  disruption  of  rock.  In  approaching 
and  passing  over  soil  and  loose  rock  the  ice  at  times  may  push  some  of 
the  unconsolidated  materials  along  near  the  ice  front,  but  in  most  cases 
the  ice  freezes  to  these  materipls  and  carries  them  along  in  a  mixture  of 
ice  and  debris.  Indeed,  where  the  heavily  loaded  basal  portions  of  the 
great  continental  glaciers  in  Greenland  have  been  observed,  it  is  noted 


198  GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


FIG.  178. — Mt.  Stephens,  with  smoothed  glaciated  lower  slopes  and  rugged  unglaci- 
ated  upper  slopes,  British  Columbia.     (Canadian  Geological  Survey.) 

that  there  is  often  no  sharp  distinction  between  frozen  ground  and  the 
abundant  debris  imbedded  in  the  moving  ice,  Fig.  179.     The  moving 

glacier  plucks  off 
projecting  rock  and 
moves  around  low 
hills,  sweeping 
away  their  soil  and 
loose  rocks  and 
adding  them  to  its 
load;  such  hills 
are  usually  steeper 
where  the  ice  im- 
pinges in  its  for- 
ward movement 
(the  stoss  side)  and 
more  gentle  on  the 
opposite  side  (the 
FIG.  179. — Debris  accumulated  beneath  a  continental  glacier  lee  side),  Fig.  180. 
in  Greenland.  (U.  S.  Geological  Survey.)  Another  source  of 

material  for  many 

mountain   glaciers,  which  lie  in  valleys  or  are  overlooked  by  cliffs,  are 
the  rocks  which  fall  to  the  surface  of  the  glacier.     Such  accumulations, 


THE  WORK  OF  GLACIERS 


199 


FIG.  180.— Stoss  side  (left)  and  lee  side 
(right)  of  a  hill  being  abraded  beneath 
a  glacier.  The  arrows  show  the  direction 
of  ice  movement. 


during  the    long    journey  of    the  glacier,  may  become  considerable. 

They  naturally  accumulate  near  the  sides  of  the  glacier,  forming  lateral 

surface  moraines  and,  when  two 

or    more    glaciers   unite   these 

moraines    are   often   borne   on 

the  surface    as    long    lines   of 

debris,  Fig.  181.      In  addition 

to  the  debris  near  the  top  and 

bottom   of  a  glacier,   there   is 

more  or  less   debris   embedded 

in  the  other  parts  of  the  glacier. 

The    winds     also     blow   small 

amounts  of  dust  to  the  surface  of  some  glaciers.     By   the  process  of 

ablation,  or  melting  down  of  glaciers,  the  surface  debris  is  sometimes 

thickened  so  that,  in  their  lower  portions,  some  glaciers  are  covered 

with  debris  to  depths  of  several  feet  and  this  debris  may  even  support 

a  heavy  growth  of  trees. 

A  glacier  acts  as  a  huge  mill  grinding  its  load  to  smaller  and  smaller 

sizes  so  that  much  of  the  material  deposited  by  ancient  glaciers  is 

composed  of  clay.  The  attri- 
tion of  pebbles,  sand  and 
stones  upon  each  other  and 
upon  the  rock  floor  reduces 
many  of  them  to  fine  powder 
so  that  milky  streams  flow 
from  many  glaciers,  the  color 
being  due  to  the  fine  rock 
flour  which  the  streams  carry 
from  beneath  the  ice.  A 
further  agency  in  comminuting 

FIG.  181. — Long  lines  of  surface  moraines  on  a  the   ice   load    is   the   shearing 
glacier,  Alaska.     (U.  S.  Geological  Survey.)      movements   in   the   glacier  by 

which    different    layers    slide 

over  each  other  and  thus  rend  and  grind  the  load.     Some  rocks  at 

the  top  of  the  glacier  may  be  repeatedly  frozen  and  thawed  and  thus 

weakened.     It  is    significant    of  the  rough    handling    to    which    the 

glacier  load  is  subjected  that  most  of  the  surviving  boulders  are  hard 

crystalline  rocks  like  granites,   quartzites  and  strong  limestones  and 

sandstones.     Shales  and  weak  limestones  and   sandstones  are  seldom 

found  in  glacial  debris. 


200 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


FIG.  182. — Perched  boulder  of  quartzite  resting 
on  marble;  an  erratic.  (Dale,  U.  S.  Geo- 
logical Survey.) 

183,  are  frequent  in  areas  with  numer- 
ous erratics.  Geologically,  erratics 
are  of  great  value  in  tracing  move- 
ments of  the  ice.  "  Perched  boul- 
ders," "  balanced  rocks,"  "  rocking 
stones "  are  expressive  terms  for 
some  erratics. 


GLACIAL  DEPOSITION 

Fortunately  for  mankind  in  the 
glaciated  regions,  much  of  the  ice- 
eroded  surfaces  has  been  recovered 
by  the  ice  as  the  glaciers  retreated. 
In  places  this  covering  is  very  thin 
and,  again,  it  is  hundreds  of  feet  in 
thickness.  These  deposited  materials 


Erratics  are  foreign  boul- 
ders which  have  been  taken 
up  or  plucked  and  then  car- 
ried and  deposited  by  the 
ice;  they  include  weak  boul- 
ders, which  have  been  carried 
only  a  short  distance,  but 
erratics  for  the  most  part 
are  strong  rocks  which  have 
endured  the  wear  and  tear  of 
the  journey  of  the  ice.  In 
the  Middle  West  these 
boulders  of  granite,  diorite 
and  other  rocks  attract  con- 
siderable attention,  for  they 
are  so  different  from  the 
underlying  rocks;  they  are 
frequently  used  for  building 
materials.  In  New  England 
they  are  locally  so  numerous 
that  clearing  them  away  is 
an  important  agricultural 
problem.  Stone  fences,  Fig. 


FIG.  183. — Stone  fences  of  glacial 
rocks,  Wis.  (Alden,  U.  S.  Geologi- 
cal Survey.) 


THE   DRIFT 


201 


are  naturally  of  great  agricultural  interest,  since  they  furnish  the  soils 
of  the  glaciated  regions. 


The  Drift 

Drift  is  the  general  term  applied  to  all  glacial  debris,  a  term  that 
was  used  when  it  was  believed  that  glacial  materials  were  deposited  by 
water.  The  drift  may  be  stratified  when  it  is  due  largely  to  water  work 
or  unst ratified  when  due  largely  to  ice  deposition.  Till  is  the  term 
applied  to  the  heterogeneous  mass  of  clay,  pebbles,  sand  and  boulders 
deposited  by  the  ice,  Fig.  184;  the  more  descriptive  term,  boulder  day,  is 
often  used  for  this  unassorted 
material.  The  most  charac- 
teristic feature  of  the  drift  is 
its  heterogeneity.  In  a  small 
area  one  may  find  stratified 
and  unstratified  materials; 
variations  in  size  from  boul- 
ders through  gravel  and  sand 
to  clay  together  with  rocks  of 
many  origins,  different  com- 
positions and  both  fresh  and 
weathered  rocks.  In  short, 
the  drift  as  a  whole  is  that 
unassorted,  heterogenous  mass 
of  rocks,  clay  gravel,  etc.,  with 
clay  usually  predominating 
that  one  would  expect  from 
such  an  agent  as  ice.  It 

should,  however,  be  remembered  that  considerable  areas  of  local  drift 
are  fairly  homogeneous  for  some  distances  and  some  glacial  soils 
are  as  homogeneous  as  many  residual  soils,  but,  as  compared  with 
alluvial  soils  and  with  most  residual  soils,  glacial  materials  and  soils 
are  decidedly  changeable.  The  heterogeneity  of  glacial  materials 
arises  from  the  complexity  of  their  origin,  the  variety  of  the  ice  load 
and  the  different  processes  involved  in  the  formation  of  drift. 

The  thickness  of  the  drift  shows  an  expectable  variation  from  zero 
to  scores  or  hundreds  of  feet,  the  thickness  varying  with  the  amount 
originally  deposited,  the  erosion  subsequent  to  the  deposition  and  to  the 
preglacial  topography,  Fig.  185.  It  is  not  unusual  in  the  upper  Mis- 


FIG.  184.— Glacial  till  lying  on  solid  rock,  N.  J. 
(U.  S.  Geological  Survey.) 


202 


GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 


isssippi  Basin  for  large  areas  to  be  so  thickly  covered  with  drift  that  the 
underlying  rock  is  not  exposed  even  in  the  deepest  valleys.  The  state- 
ment may  be  made,  although  there 
are  many  exceptions,  that  in 
rugged  regions  like  New  England 
the  drift  is  thin,  and  in  smooth 
regions  like  the  plains  of  the  upper 
Mississippi  Basin  it  is  thick.  In 


FIG.  185. — Diagram  to  illustrate  varia- 
tion of  drift  thickness  due  to  buried 
hills  and  valleys. 


general,  some  till  is  found  wherever 
there  was  ice,  but  the  stratified 

drift  was  carried  beyond  the  glaciers  so  that,  as  a  result,  the  area  of 
drift  is  larger  than  the  area  covered  by  the  ice. 

Composition  of  Drift. — The  variations  both  of  materials  and  of 
their  arrangement  in  the  drift  are  especially  characteristic.  In  a  single 
exposure  one  may  often,  find  variations  in  size  from  boulders  to  clay, 
but  even  the  most  stony  till  usually  contains  considerable  percentages 
of  fine  materials.  Two  examples  will  show  this  variation.  From  a 
careful  study  of  the  somewhat 
stony  drift  around  Boston,  Cros 
by  has  estimated  that  the  clay 
or  rather  rock  flour  constitute, 
about  10  per  cent  of  the  drift 
while  nearly  50  per  cent  is  com- 
posed of  sand.  In  contrast, 
Leverett  found  that  some  of  the 
till  in  Illinois  contains  over  90 
per  cent  of  clay. 

The  size  of  the  different  con- 
stituents of  till  is  dependent  on 
many  factors.  Obviously  the 
resistance  of  the  rocks  deter- 
mines to  a  considerable  degree 
their  endurance  of  the  wear  and 


FIG.  186. — Glacial  clay.  Note  the  angular 
bits  of  rock  scattered  through  the  clay 
(much  magnified). 


tear  of  glacial  transportation  so 

that  the   boulders  and  much  of 

the  smaller  materials  are  granite, 

quartzite  and  other  strong  rocks  that  may  be  carried   hundreds  of 

miles.     For  example,  quartzite  boulders,  locally  called  "  lost  rocks," 

are  rather  common  in  the  drift  on  northern  Missouri,  and  these  have 

been  transported  several  hundred  miles  with  but  little  evidence  of  wear. 


THE  DRIFT 


203 


Most  shales  are  so  weak  that  they  are  for  the  most  part  broken  up 
when  they  are  first  acquired  by  the  ice  and  are  rarely  transported  for 
any  considerable  distance  except  as  clay,  silt  and  small  fragments. 
Many  schists  and  some  gneisses,  sandstones  and  limestones  are  also 
ground  into  rock  flour  by  the  ice. 

It  should  always  be  remembered  that  glacial  clay  and  residual  clay  are  different; 
they  differ  both  chemically  and  physically.  Glacial  clays  are  ground-up  rocks  for 
the  most  part  although  the  ice  of  course  picks  up  and  transports  residual  clays,  while, 
on  the  other  hand,  residual  clays  are  the  insoluble  clays  left  after  long  weathering. 
Glacial  clays  under  the  microscope  often  show  many  minute  angular  fragments 
of  rocks  and  minerals,  usually  fairly  fresh,  while  residual  clays  usually  show  weath- 
ered and  more  or  less  rounded  minerals,  usually  much  decomposed,  Fig.  186.  Under 
the  same  conditions  drift  is  fresher,  less  weathered  and  less  leached  than  the  same 
kinds  of  residual  materials,  as  the  following  table  illustrates:1 


1  (residua'  clay) 

% 

2  (Glacial  lake  clay). 

% 

Silica                           

62  11 

44  51 

Alumina 

16  96 

8  00 

Phosphoric  acid  

00  025 

00  09 

Iron  oxides                        .                  

7  65 

3  19 

Lime  (CaO)                                  ' 

90 

13  74 

Magnesia  '.  

90 

7  42 

Potash 

1  22 

2  48 

Soda  

1.82 

88 

Carbon  dioxide  

20 

17  11 

1  Chamberlin  and  Salisbury,  6th  Annual  Report,  U.  S.  Geological  Survey,  page  250. 

Number  1  is  a  composite  sample  of  residual  clays  and  Number  2  includes  samples 
of  glacial  lake  clays.  The  samples  are  not  to  be  regarded  as  characteristic  except 
that  they  show  clearly  the  greater  leaching  of  the  residual  clays.  The  variety  of 
materials  in  the  drift  is  readily  understood  when  it  is  remembered  that  the  glaciers 
carried  preglacial  soil  and  subsoil,  weathered  and  fresh  rocks,  alluvial  materials  and 
not  infrequently  even  older  glacial  materials.  Furthermore,  all  these  were  subjected 
to  the  destructive  processes  due  to  the  ice  acquiring  its  load  and  to  the  wear  of  the  load 
during  transit. 


Moraines 

Glaciers  deposit  mainly  in  two  places,  beneath  the  glacier  and  at  or 
near  its  margin.  Ice,  like  water,  may  become  overloaded  with  debris 
and  be  compelled  to  deposit  some  of  its  load  beneath  the  glacier  as  it 
advances.  Such  deposition  beneath  the  ice  occurs  only  locally  and, 


204  GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 

furthermore,  such  deposits  are  exposed  to  the  erosion  of  the  ice  following 
after;  in  fact  these  deposits  beneath  the  ice  are  subject  to  repeated 
erosion  and  deposition.  Such  deposits  are  termed  ground  moraine; 
on  the  whole  they  are  not  extensive.  In  places  this  ground  moraine 
constitutes  the  subsoil  and  is  usually  known  as  "  hard  pan";  it  is  often 
much  compacted  by  the  pressure  of  the  overriding  ice  and  must  be 
removed  by  dynamiting  when  wells  are  dug  through  it. 

Terminal  Moraines 

Conditions  for  deposition  are  exceptionally  favorable  when  the 
margin  of  the  ice  becomes  relatively  stationary  because  of  melting  or 
evaporation.  The  ice  under  such  conditions  is  inactive  and  sluggish, 
less  able  to  carry  its  load  and  more  likely  to  deposit  debris  beneath 
the  glacier.  It  is  of  course  obvious  that  melting  ice  at  once  drops  its 
load.  It  has  been  noted  before  that,  even  when  the  glacier  as  a  whole 
is  advancing,  it  is  subject  to  halts,  retreats  and  readvances  and  the  same 
is  true  when  it  is  retreating.  When  the  rate  of  ice  advance  is  about 
balanced  by  the  rate  of  melting,  the  drift  is  deposited  near  the  margin 
and  forms  the  terminal  moraine.  If  the  ice  front  is  nearly  stationary 
for  a  long  time,  a  well-marked  ridge  would  be  built,  more  or  less  coin- 
ciding with  the  front  of  the  glacier;  in  a  comparatively  few  places  wall- 
like  or  ridge-like  terminal  moraines  have  been  built,  but  they  are  on 
the  whole  exceptional.  However,  the  usual  oscillations  of  the  ice  front 
scatter  the  drift  over  a  zone  several  miles  in  width  so  that  the  usual 
terminal  moraine  is  a  low,  broad  ridge  composed  of  drift  and  it  may  be 
indistinct  in  places.  Terminal  moraines  are  characteristically  composed 
of  till,  but  it  is  readily  apparent  that  they  also  contain  much  stratified 
drift  since  they  were  usually  deposited  in  a  zone  of  melting  ice.  • 

The  topography  of  a  terminal  moraine  is  usually  somewhat  char- 
acteristic. Typically  it  consists  of  small  rounded  hills  and  adjacent 
depressions  in  a  disorderly  arrangement,  Fig.  187.  Many  of  the  depres- 
sions are  kettle-like  in  shape  and  consequently  they  are  often  locally 
called  "  kettle  holes";  where  these  depressions  occur  in  till  they  often 
contain  water,  and  dozens  of  small  ponds  often  map  a  terminal  moraine. 
Again  a  terminal  moraine  is  often  composed  largely  of  low  interlocking 
ridges  of  drift.  The  following  terms  are  somewhat  descriptive  of  this 
peculiar  topography  and  are  in  common  use:  "  sag  and  swell"; 
"knob  and  basin";  "hummocky";  "hummocks  and  hollows."  In 
places  the  surface  is  decidedly  hilly  but  more  commonly  it  consists  of 


TERMINAL   MORAINES  205 

gentle  swells  with  low  interlying  depressions,  all  giving  an  undulating 
surface. 

It  is  not  always  easy  to  explain  this  topography.  In  places  the  ice 
carried  and  deposited  a  heavier  load  and  here  would  naturally  be  an 
elevation.  Ice  blocks  become  detached,  covered  with  debris  and,  upon 
melting,  may  leave  a  depression.  Then  the  ice  advancing  and  retreating 
in  minor  movements,,  pushing  the  old  drifts  and  depositing  new  drift, 
would  account  for  some  of  the  peculiar  topography: 


FIG.  187. — Terminal  moraine  on  a  valley  side,  N.  Y.     (Tarr,  U.  S.  Geological  Survey.) 

Recessional  Moraines. — It  will  be  remembered  that  both  the  ice 
advance  and  the  ice  retreat  were  characterized  by  halts  and  at  each 
prolonged  halt  the  glacier  built  a  terminal  moraine.  The  moraines 
built  by  the  advancing  ice  were  overridden  and  for  the  most  part,  oblit- 
erated. On  the  other  hand,  as  the  ice  front  retreated,  it  built  moraines 
during  the  various  halts  which  are  often  termed  recessional  moraines, 
which  are  in  all  respects  like  terminal  moraines.  The  glacier  front,  and 
consequently,  the  terminal  moraine,  were  almost  never  straight,  but  in 
lobes,  and  this  arrangement  is  especially  emphasized  where  lowlands  and 
valleys  extended  in  the  general  direction  of  ice  movement,  the  lobes  of 
ice  extending  down  valleys.  This  is  well  shown  in  the  great  reach  of 
country  extending  from  Wisconsin  to  New  York,  where  the  lobes  extended 
in  a  southerly  direction  along  the  Great  Lake  depressions,  Fig.  188. 
The  same  features  are  seen  in  the  Allegheny  Plateau  of  southern  New 
York,  where  many  recessional  moraines  are  ranged  in  lobes  one  behind 


206 


GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 


the  other  in  the  north-south 
valleys,  while  the  moraines  are 
very  faint  on  the  uplands. 

The  soils  of  terminal  and 
recessional  moraines  are 
varied  in  composition,  texture 
and  drainage,  but  texture  and 
drainage  show  the  greatest 
variation  in  short  distances. 
The  water  derived  from  the 
melting  ice  at  the  glacier's 
margin  naturally  produced 
variations  in  texture  so  that 
small  areas  of  sandy,  heavy 
and  medium  textures  are  often  closely  intermingled.  On  the  other 
hand,  the  surface  drainage  is  dependent  on  the  topography,  which 
varies  from  well-drained  hil- 
locks to  the  undrained  sags. 


FIG.  188. — The  dotted  lines  mark  successive 
positions  of  the  ice  in  its  retreat.  (After 
Leverett,  U.  S.  Geological  Survey.) 


Fig.  189  shows  the  varied  soils 
of  a  terminal  moraine  in  southeast- 
ern Michigan.  The  clay  loams  are 
derived  from  till  which  contains 
some  sand  while  the  gravelly  and 
sand-loams  are  derived  from  water 
laid  materials,  the  one  from  rapid 
and  the  other  from  moderate  cur- 
rents. The  muck  soils  and  the  lakes 
occupy  low  undrained  depressions. 
Not  all  soils  of  terminal  and  reces- 
sional moraines  are  so  complicated, 
but  usually  these  soils  are  more 
complicated  than  the  soils  of  other 
glacial  features. 

The  Ground  Moraine 

When  a  glacier  recedes,  its 
debris,  instead  of  being  con- 
centrated in  a  terminal  "mo- 
raine, is  scattered  over  the 
surface  left  by  the  retreating 

ice.     Such  a  deposit  is  termed  the  ground  moraine,  a  term,  it  will  be 
remembered,  that  is  also  applied  to  the  far  less  important  deposits 


FIG.  189. — Morainic  soils,  Mich.     (Data  after 
U.  S.  Bureau  of  Soils.) 


THE  GROUND  MORAINE 


207 


beneath  the  advancing  glacier.  The  ground  moraine  is  by  far  the 
most  important  and  widespread  deposit  of  the  great  continental  gla- 
ciers, at  least  from  an  agricultural  point  of  view.  It  is  characteris- 
tically composed  for  the  most  part  of  till,  but  it  also  contains  varying 
amounts  of  stratified  drift.  In  general  the  terminal  moraine  contains 
a  higher  percentage  of  boulders  and  much  more  intermingled  stratified 
drift  than  the  ground  moraine.  The  topography  of  the  ground  moraine 
is  commonly  smooth  or  gently  rolling,  Fig.  190,  except  where  modified 
by  later  erosion.  In  places,  as  in  north-central  Ohio,  it  is  so  smooth  as 
to  be  a  typical  plain;  the  glacial  prairies  of  Illinois,  Iowa  and  northern 
Missouri  are  largely  ground  moraine.  Such  a  youthful  topography  is 
likely  to  be  poorly  drained  both  because  of  the  depressions  in  which 


FIG.  190.— Ground  moraine,  Wis.     (W.  J.  Geib,  U.  S.  Soil  Survey.) 

water  may  stand  and  because  the  streams  are  not  yet  organized  for 
effective  drainage  and  erosion,  Lakes,  ponds  and  swamps  are  rather 
characteristic  of  the  ground  moraine,  although  they  are  nearly  absent 
on  some  of  the  older  drifts  where  streams  are  well  established  and  on 
some  of  the  well-drained  portions  of  the  newer  drift.  A  ground  and 
terminal  moraine  are  shown  in  Fig.  196. 

The  soils  of  ground  moraines  are  less  variable  than  those  of  terminal 
moraines  and  the  ground  moraine  soils  tend  towards  clays,  clay  loams 
and  silt  loams  rather  than  the  lighter-textured  soils.  Compared  with 
terminal  recessional  moraines,  these  soils  are  relatively  free  from  stones. 
The  relatively  level  topography  produces  somewhat  poor  drainage  so 
that  many  thousand  miles  of  tile  drainage  have  been  installed  on  the 
ground  moraines  of  the  Middle  West.  The  level  surface  with  the  result- 


208  GLACIERS  AND  GLACITAION;   GLACIAL  SOILS 

ing  slow  drainage  has  also  promoted  an  accumulation  of  humus  in  these 
soils.  Many  of  the  more  shallow  depressions  are  occupied  by  muck 
soils. 

Drumlins 

These  are  hills  that  are  sometimes  associated  with  the  ground 
moraine.  Their  ground  plan  is  in  general  somewhat  ovoid,  their  longer 
axis  lies  parallel  to  the  general  direction  of  ice  movement  and  their 
steeper  slopes  are  generally  to  the  northward,  from  which  direction  the 
ice  advanced.  Beyond  the  fact  that  drumlins  are  distinctively  a  glacial 
product,  their  origin  is  as  yet  undecided.  They  usually  occur  in  groups 


FIG.  191.— "Side"  View  of  a  Drumlin,  Wis.     (Alden,  U.  S.  Geological  Survey.) 

and  are  often  locally  called  "  whalebacks."  Drumlins  are  composed 
largely  of  till  and  they  contain  but  little  stratified  drift.  Their  soils 
are  rather  heavy  and  naturally  subject  to  erosion  because  of  their  fine 
texture  and  the  steep  slopes  on  which  they  lie.  Areas  between  drumlins 
usually  have  poor  drainage  and  often  the  soils  contain  considerable 
humus ;  in  some  parts  of  New  York  such  soils  have  been  used  for  celery, 
peppermint  and  other  truck  crops.  Drumlins,  themselves,  because  of 
their  steep  slopes,  are  usually  left  in  pasture. 

Relations  of  Drift  and  Glacial  Soils  to  Local  Formations 

Introductory. — While  it  is  probably  true  that  each  square  mile  of 
drift  contains  materials  from  all  the  surface  over  which  the  contributing 
ice  has  passed,  yet,  as  a  whole,  the  drift  shows  more  or  less  close  rela- 
tions to  the  adjacent  underlying  rocks.  This  relation  is  usually  closer 


DRUMLINS  209 

where  the  drift  is  thin  rather  than  thick;  indeed,  where  the  drift  is  very 
thick  there  is  often  no  apparent  relation  whatever.  It  is  estimated 
by  Chamberlin  and  Salisbury  that,  on  the  whole,  75  per  cent  of  the  drift 
has  not  been  carried  more  than  50  miles.  Shaler  estimates  that,  in  the 
Narragansett  Basin  of  southern  Massachusetts  and  eastern  Rhode 
Island,  about  80  per  cent  of  the  drift  is  local  and  Alden,  working  in 
eastern  Wisconsin,  made  a  careful  study  showing  that  at  least  87  per 
cent  of  the  drift  is  local.  These  estimates 
made  after  long  study  clearly  show  that  usually 
there  is  a  close  relation  between  the  drift 
and  the  neighboring  underlying  rocks. 


The  local  origin   of   some   drift   is   illustrated  by 
Fig.  192.     A  small  area  (S)  in  northern  New  Jersey 
of  a  distinctive  rock   (syenite)   has  been  .thoroughly 
eroded  by  the  ice  and  the  debris  has  been  scattered 
over  a  fan-like  area  for  several  square  miles.      It  is 
found  that  these  characteristic  rocks  are  smaller,  more 
scattered  and  less  plentiful  with  increasing  distance      FIG.  192. — Fan-like  glacial 
from  the  parent  outcrop  (£).     If  we  imagine  not  one          debris  from  outcrop  (£). 
outcrop  which    is    eroded   by    the    moving    ice,    but.        (Salisbury,  N.J.  Geolo- 
thousands,  it  is  clear  that  much  of  the  drift,  must  be         8ical  Survey.) 
local.     Many  factors,  of  course,  such  as  the  resistance 

of  the  rock  and  the  vigor  of  erosion  are  influential  in  determining  the  proportion 
of  local  drift  in  a  given  locality. 

Influence  of  Local  Rocks  on  Glacial  Soils. — The  local  bed  rocks 
usually  show  more  or  less  influence  on  glacial  soils.  Glacial  soils  over- 
lying sandstones  are  likely  to  be  sandy  or  loams;  those  overlying 
shales  are  likely  to  be  heavy;  soils  overlying  limestones,  in  contrast 
with  most  residual  limestone  soils,  are  very  often  highly  calcareous, 
so  much  so  that  subsoils  and  often  soils  will  effervesce  with  acid;  glacial 
soils  overlying  granites  are  likely  to  contain  many  granite  fragments, 
they  are  often  somewhat  stony  and  usually  show  a  somewhat  high  con- 
tent of  potash.  Glacial  soils  sometimes  resemble  true  residual  soils 
and  this  is  especially  true  of  heavy  glacial  soils  derived  from  shales. 
Stony  glacial  soils  show  a  close  relation  to  the  character  of  the  local 
bed  rock  over  which  the  glaciers  passed  and  to  the  distance  the  ice  load 
has  been  transported.  If  the  underlying  or  adjacent  rocks  are  resistant 
as,  for  example,  granites  or  quartzites,  the  till  and  soil  are  likely  to  be 
stony. 

The  influence  of  bed  rock  on  glacial  soils  is  especially  noticeable 
when  the  bed  rock  is  somewhat  distinctive,  especially  in  color.  This  is 


210 


GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 


well  shown  in  the  Connecticut  Valley  of  Massachusetts  and  Connecticut 
where  the  underlying  red  shales  and  sandstones  impart  a  reddish  color 
to  much  of  the  drift  and  often  to  the  subsoils.  Another  example  is 
found  in  the  Volusia  soils,  which  are  widespread  and  somewhat  charac- 
teristic of  the  northern  portions  of  the  Allegheny  Plateau.  The  under- 
lying rocks  are  largely  of  shales  and  sandstones.  Glaciation  in  this  hilly 
region  was  relatively  ineffective  in  changing  the  character  of  the  soils 
and  the  Volusia  soils  differ  but  little  from  residual  soils  on  the  same 
formations  further  south  beyond  the  limits  of  glaciation. 

An  excellent  study  of  soil  values  as  determined  by  the  influence  of 
local  rock  on  drift  and  soils  has  been  made  by  Whitbeck.1  In  south- 
eastern Wisconsin  there  is  a  large  area  not  covered  with  drift  and 
adjoining  this  are  areas  in  which  the  drift  is  underlain  by  sandstone  and 
limestone  with  the  usual  result  that  the  drift  and  its  derived  soils  are 
markedly  influenced  by  local  bed  rocks.  Some  of  his  conclusions  are 
as  follows: 


DRIFTLESS  AREA. 

GLACIATED  AREA. 

Sandstone. 

Limestone. 

Sandstone. 

Limestone. 

Per  cent  of  improved  land  
Average  values  of  all  crops  per  square  mile  .  . 

37.2 

$1,968 

60.5 
$2,690 

48.2- 

$2,776 

70.3 

$3,828 

"  The  yield  of  corn  oats  and  barley  was  a  little  higher  per  acre  in 
the  limestone  belt  but  only  a  little  higher  in  the  drift  soil  than  in  the 
driftless.  In  the  sandstone  belt  the  difference  is  more  pronounced. 
In  each  of  these  crops,  the  average  yield  per  acre  is  distinctly  higher  in 
the  drift  soil,  averaging  in  the  three  crops  practically  33J  per  cent. 
The  residual  limestone  soil  of  Wisconsin,  like  that  of  Kentucky  or 
Virginia,  is  inherently  rich  and  would  not  be  much  improved  by  the 
addition  of  the  drift.  The  residual  sandstone  soil  is  inherently  sterile 
and  would  be  materially  improved  by  the  addition  of  drift  which  came 
from  the  limestone  area  on  the  east." 

An  interesting  area  in  northeastern  Wisconsin  illustrates  a  relation  of  glacial  soils 
to  underlying  granite,  sandstone  and  limestone,  Fig.  193.  The  soils  above  the 
granites  are  somewhat  stony  and"  sandy  with  many  scattered  granite  boulders  and 
rocks.  The  magnesian  limestone  contains  considerable  sand  and  the  soil  above  this 
and  above  the  sandstone  as  well  is  a  fine  sandy  loam  but,  as  one  would  expect,  the 

1  Aspects  of  the  Glaciation  of  Wisconsin,  R.  H.  Whitbeck,  Annals  of  the  Asso- 
ciation of  American  Geographers,  Vol.  3,  1913. 


DRUMLINS 


211 


soils  over  the  limestone  contain  many  limestone  boulders  and  fragments.  The  drift 
above  the  Trenton  limestone  contains  a  considerable  admixture  of  materials  from  the 
granite  and  sandstone  areas  to  the  northward,  but  the  soil  is  a  somewhat  calcareous 
sandy  loam  and  the  subsoil  is  largely  composed  of  limestone  debris. 


FIG.  193. — Diagram  to  illustrate  some  relations  between  glacial  soils  and  the  underr 
lying  rocks  in  Wisconsin.     (Data  after  U.  S.  Bureau  of  Soils.) 

Ice  Movement  and  Rock  Strike. — The  relation  between  the  direc- 
tion of  ice  movement  and  the  direction  of  rock  outcrop  (Strike,  page  58) 
is  important.  When  a  glacier  moves  for  a  considerable  distance  along 
a  rock  outcrop,  the  ice  load  is  necessarily  derived  largely  from  the 
outcrop,  the  drift  will  contain  much  material  from  the  outcrop  and 
the  soils  will  considerably  resemble  residual  soils  from  this  outcrop. 
On  the  other  hand,  when  a  glacier  moves  across  several  rock  outcrops, 
the  drift  will  contain  varying  amounts  from  the  different  outcrops  and 
the  resultant  soils  are  likely  to  be  a  mixture  of  materials  of  far  greater 
complexity  than  in  the  other  instance. 

A  case  in  point  is  shown  in  Fig.  194.  This  region  in  northwestern  New  Jersey  is 
somewhat  mountainous;  the  rocks  are  folded  and  in  consequence  there  is  a  sue- 


CULVER 
STONY  LOAM 


DOVER        /GLOUCESTER! 


K"*  ",  GRANjtE  »  V»r| 

-.GNEISS';  »  *  » 


FIG.  194. — Generalized  diagram  to  illustrate  the  relations  of  rocks  and  soils  when 
the  glacial  movement  was  parallel  to  the  rock  outcrops.  (Data  after  U.  S. 
Bureau  of  Soils.) 

cession  of  ridges  and  valleys  (see  Fig.  57).     The  general  ice  movement  was  from 
northeast  to  southwest  roughly  parallel  "with  the  rock  outcrops  with  the  result  that 


212  GLACIERS  AND  GLACIATION:   GLACIAL  SOILS 

the  till  and  its  derived  soils  show  a  banded  arrangement  such  as  we  find  in  residual 
soils  from  folded  rocks  (Fig.  49).  In  the  eastern  belt  the  Gloucester  soil  series  is 
derived  largely  from  gneisses;  it  is  sandy  and  likely  to  contain  considerable  unde- 
composed  mica  and  gneissic  rocks  of  varying  size.  The  limestones  give  a  cal- 
careous till  from  which  are  derived  the  Dover  soils  which  are  mainly  loams  often  con- 
taining limestone  fragments  of  various  sizes.  The  Dutchess  soils  are  derived  largely 
from  till  which,  for  the  most  part,  was  gathered  from  shales  and  slates;  slate  frag- 
ments are  very  common  in  the  drift  which  is  usually  rather  thick.  The  Culver 
stony  loam  is  derived  from  a  somewhat  mixed  till;  the  rock  fragments  and  boulders 
are  clearly  derived  from  the  conglomerate  and  sandstone  of  the  adjacent  rock  belt 
to  the  westward  and  the  finer  materials  of  the  till  are  derived  from  the  immediately 
underlying  shales  and  slates  and  from  a  sandstone  belt  to  the  westward.  Much  of 
this  belt  is  rough  stony  land. 

As  a  whole  the  topography  of  this  region  is  rough,  ranging  from  mountain  ridges 
to  hills.  The  ice  in  most  of  the  region  eroded  the  preglacial  soil  and  failed  to  replace 
its  equivalent  and,  furthermore,  the  hilly  topography  has  favored  erosion  by  which  the 
glacial  soils  have  been  in  part  washed  away  since  they  were  deposited.  The  Wall- 
pack  soils  which  form  the  westernmost  soil  belt  show  another  not  uncommon  feature 
of  glacial  soils  in  hilly  regions.  The  soils  are  in  part  derived  from  limestone,  sand- 
stone and  shale  till,  but  some  are  true  residual  soils,  being  derived  from  post  glacial 
weathering  of  exposed  weak  rocks.  An  examination  of  the  diagram  shows  that  soil 
boundaries  and  rock  boundaries  are  far  from  coinciding,  much  less  coincidence,  in 
fact,  than  we  find  in  residual  soils.  There  were  different  minor  ice  movements 
sometimes  at  angles  to  the  main  glacial  movement,  movements  which  carried  the 
debris  across  the  formations. 


FLUVIO-GLACIAL  WORK 

For  convenience  of  discussion  we  have  considered  the  work  of  ice 
by  itself  without  reference  to  the  water  which  was  usually  associated 
with  the  glaciers  even  in  their  advance  and  which  was,  of  course,  a  very 
prominent  factor  when  the  ice  was  melting  back.  It  has  been  seen 
that  the  advancing  continental  glaciers  were  subject  to  halts  and  even 
minor  retreats  due  to  the  melting  of  the  ice.  At  such  times  the  water 
from  the  melting  ice  made  stratified  deposits  most  of  which  were  de- 
stroyed when  the  glacier  resumed  its  main  advance.  Nearly  all  drift 
contains  "  pockets  "  of  stratified  materials  which  were  formed  by  water 
derived  from  local  melting  of  the  ice.  But  the  greatest  floods  of  water 
coming  from  melting  ice  came  when  the  glaciers  made  long  halts  or  were 
in  retreat  when  enormous  quantities  of  water  were  released,  waters 
which  took  up  and  redeposited  the  drift.  Such  deposits  made  by 
water  from  melting  glaciers  are  termed  fluvio-glacial  deposits.  They 
are  especially  important  from  the  standpoint  of  soils  because  they  are 
usually  surface  materials  from  which  large  areas  of  soils  are  derived. 


OUTWASH  PLAINS  213 

Fluvio-glacial  soils  are  characteristically  coarse-textured  because  they 
are  usually  deposited  by  rather  swift  currents  with  the  result  that 
gravelly  and  sandy  soils  are  rather  characteristic  of  deposits. 

Outwash  Plains 

When  a  glacier  halts  on  a  plain  sloping  away  from  the  ice,  the  waters 
from  the  melting  ice  spread  over  the  plain,  cover  it  with  their  deposits 
and  build  an  outwash  plain.  Usually  the  water  from  the  ice  escapes 
in  streams  flowing  through  and  away  from  the  terminal  moraine  rather 
than  as  a  broad  sheet  of  water  and  these  streams  each  build  low  alluvial 
fans  which  coalesce  into  a  compound  fan,  the  whole  series  of  fans  making 
the  outwash  plain.  Fig.  195 
shows  a  stream  flowing  from 
a  glacier  in  Alaska  and  build- 
ing a  low  fan;  during  the 
glacial  period  thousands  of 
these  streams  built  the  wide 
stretches  of  outwash  plains. 

The   swift,    heavily   laden 
waters     built    steeper    slopes      FlG  i95._Glacial  streams  building  alluvial 
near   the    terminal    moraines         fans,    Alaska.     (Tarr,    U.   S.    Geological 
and   also  here  deposited  their         Survey.) 
heavier    loads,    often   gravels 

mixed  with  sand.  Farther  away  from  the  moraine  the  deposits 
were  built  into  more  gentle  slopes  and  the  materials  deposited  were 
finer.  A  typical  outwash  plain  has  a  very  flat  surface  in  the  outer 
portions  away  from  the  moraine,  so  level,  indeed,  that  such  portions 
are  often  locally  called  "  prairies."  Nearer  the  moraine  the  slopes  rise 
and  the  topography  is  likely  to  become  "  ridgy."  Not  infrequently 
the  surface  is  interrupted  by  depressions  and  ponds  where  the  waters 
built  debris  about  masses  of  ice  which  had  become  detached  from  the 
glacier's  front  and  this  ice  upon  melting  left  depressions.  Usually  some- 
what faint,  shallow  valleys,  now  often  unoccupied  by  streams,  show  the 
old  courses  of  streams  flowing  from  the  ice.  Fig.  196  shows  relations 
between  the  outwash  plain  and  terminal  moraine. 

The  soils  of  outwash  plains  usually  show  two  characteristics.  They 
are  typically  coarse-textured  sands,  sandy  loams  and  gravelly  sandy 
loams  being  frequent  types.  Again  these  soils  usually  show  a  gradation 
from  coarse-textured  soils  near  the  moraine  to  fine-textured  soils  near 


214 


GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 


GROUND   MORAINE 
AND  DRUMLINS 


SWAMP  ^.^L    JT 


SANDY  AREAS  ON  GROUND  MORAINE 


FIG.  196.  —  Diagram  showing  glacial  features  in  an  area  in  southeastern  Wisconsin. 
(Data  after  Alden,  U.  S.  Geological  Survey.) 


FIG.  197. — Looking  across  an  outwash  plain  toward  the  terminal  moraine  in  the 
background,  Maine.  The  section  is  shown  in  Fig.  252.  (U.  S.  Geological 
Survey.) 


OUTWASH  PLAINS 


215 


the  border  of  the  outwash  plain.  Drainage,  particularly  subsoil  drain- 
age, is  usually  good  because  of  the  sandy  texture  of  the  soils.  Indeed, 
the  materials  are  often  so  coarse  near  the  moraine  that  the  water  table 
stands  so  low  that  nothing  but  deep-rooted  trees  can  reach  moisture. 
Outwash  plains  are  often  fairly  well  mapped  by  crops;  the  upper  grav- 
elly portions  being  in  orchards  or  forests,  the  middle  coarse  sandy  por- 
tions being  in  truck  and  corn  and  the  lower  fine  sandy  loam  soils  are 
often  in  wheat  and  corn.  The  composition  of  these  soils  varies,  of 
course,  with  the  materials  furnished  to  the  waters  from  the  melting 
ice,  but  they  are  usually  siliceous. 

The  southern  part  of  Long  Island  includes  one  of  the  most  extensive 
outwash  plains  in  North  America.  The  surface  features  of  Long  Island 
are  almost  entirely  due  to  constructive  glaciation ;  the  Island  consists 
essentially  of  two  terminal  moraines  at  the  north  separated  by  a  narrow 
discontinuous  outwash  plain,  and  at  the  south  is  a  wide  outwash  plain 
extending  for  100  miles  and  forming  the  southern  part  of  Long  Island, 


FIG.  198. — Sketch  map  of  Long  Island,  N.  Y.,  showing  the  two  terminal  moraines 
(1  and  2)  and  the  two  outwash  plains  (3  and  4).  The  rectangle  in  the  western 
part  indicates  the  area  shown  in  Fig.  199. 

Fig.  198.  The  glacier  halted  and  built  the  southernmost  terminal 
moraine  (No.  1  in  Fig.  198)  and  the  water  from  the  melting  ice  built 
most  of  the  large  outwash  plain  (No.  3).  After  this  well-defined  ter- 
minal moraine  was  built,  a  change  of  conditions  caused  the  ice  front 
to  melt  back  over  much  of  the  Island  except  in  the  extreme  western 
part.  This  retreat  continued  for  several  miles  when  again  the  ice  front 
halted  and  built  the  second  moraine  (No.  2).  When  the  outwash  water 
built  the  second  outwash  plain  (No.  4)  but  a  part  of  the  water  escaped 
through  openings  in  the  first  terminal  moraine  (No.  1)  and  added  some 


216 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


materials  to  the  first  outwash  plain  (No.  3).  The  convergence  of  the 
terminal  moraine  in  the  western  part  of  the  Island  indicates  that  here, 
for  some  reason  not  apparent,  the  ice  front  did  not  retreat,  but  main- 
tained its  position,  while  farther  east  the  ice  front  retreated  to  the  posi- 
tion of  the  second  terminal  moraine.  Such  inequalities  in  retreat  are 
the  rule  and  not  the  exception  and  they  add  to  the  complexities  of  glacial 
studies. 

Fig.  199  shows  a  portion  of  the  first  terminal  moraine  and  the  outwash  plain  to 
the  southward  which  are  figured  in  Fig.  198.  The  surface  of  the  moraine  is  hilly  to 

rolling  without  any  definite  drainage 
lines  except  the  more  or  less  indis- 
tinct valleys  where  outwash  waters 
cut  through  portions  of  the  moraine 
that  was  already  built.  Depressions, 
some  containing  water  and  some  dry, 
show  where  ice  blocks  were  wholly  or 
partly  covered  with  debris  and  upon 
melting  left  depressions.  The  till 
contains  considerable  stratified  ma- 
terials deposited  by  water  and  there 
are  many  boulders  of  granite  and 
similar  rocks  which  the  ice  carried 
from  New  England.  There  is  in  the 
till  a  considerable  amount  of  gravel 

and  sand  which  is  largely  due  to  the  underlying  materials  which  were  in  part  taken 
up  and  incorporated  in  the  till.  The  soils  of  the  terminal  moraine  here  are  largely 
loams  and  sandy  loams  although  locally  there  are  stony  areas.  The  hills  are 
naturally  well  drained,  but  the  depressions  are  poorly  drained  except  where  the 
materials  are  so  sandy  or  gravelly  that  the  rainfall  and  inflow  are  carried  away  by 
subsurface  drainage. 

The  outwash  plain  is  seen  sloping  away  from  the  terminal  moraine  with  steeper 
slopes  nearer  the  moraine.  The  diagram  does  not  show  the  many  alluvial  fans,  some 
separate  but  many  coalesced,  which  in  many  places  extend  outward  from  the  moraine. 
In  the  upper  zone  is  a  belt  of  gravelly  loams  with  a  somewhat  lobate  margin  which 
was  deposited  by  relatively  swift  water.  This  belt  grades  southward  into  the  very 
gentle  slopes  of  the  lower  outwash  plain  where  the  soils  are  very  sandy.  In  this  part, 
when  the  outwash  plain  was  being  built,  the  streams  were  slow  and  often  divided  and 
subdivided  into  somewhat  sluggish  streamlets  which  carried  fine  materials. 

Valley  Train 

On  the  other  hand,  when  the  outwash  water  from  melting  ice  is 
spread  over  a  valley  as  the  glacier  melts  back,  the  valley  train  is  built; 
an  outwash  plain  is  built  when  the  ice  halts  for  some  time,  but  the 
valley  train  is  the  fluvio  glacial  material  scattered  over  a  region  as 


FIG.  199. — Diagram  to  illustrate  the  rela- 
tions of  soils  to  a  terminal  moraine  and 
an  outwash  plain  in  western  Long  Island. 
(Data  from  U.  S.  Bureau  of  Soils.) 


KAMES  AND  ESKERS 


217 


a  glacier  retreats.  In  a  region  traversed  by  valleys  the  ice  is  thickest 
in  valleys  and  consequently  the  principal  deposits  in  the  form  of  ter- 
minal moraines,  outwash  plains  and  valley  trains  usually  occur  in  val- 
leys which  slope  away  from  the  ice  front.  As  the  glacier  melts  back, 
the  outwash  waters,  usually  heavily  laden  with  sediment,  flow  down  the 
valley,  often  filling  it  with  gravel  and  sand  to  depths  of  scores  of  feet. 
Just  as  outwash  plains  are  analogous  to  alluvial  fans,  so  valley  train 
may  be  regarded  as  a  special  type  of  alluvial  plain,  usually  composed 
of  coarser  materials  than  a  flood  plain  and  lacking  the  front  and  back 
lands  of  flood  plains. 

Valley  trains  often  extend  far  beyond  the  farthest  limits  of  glaciation, 
whither  they  were  carried  by  outwash  waters.  In  many  cases  later 
streams  have  frequently  cut  channels  in  valley  trains,  leaving  them  as 
terraces  above  the  present  streams.  The  glaciers  in  valleys  as  elsewhere 
halted  and  built  terminal  moraines  with  accompanying  outwash  plains 
and  in  fact  there  are  all  gradations  between  outwash  plains  and  valley 
trains.  Soils  of  valley  trains 
are  usually  sandy  or  gravelly 
and  of  irregular  arrangement. 
The  valley  train  in  some  places 
is  so  thin  that  underlying  till 
constitutes  a  "  hard  pan  "  and 
wells  often  penetrate  to  the 
usually  underlying  till. 

A  somewhat  ideal  illustration  of 
these  features  is  seen  in  Fig.  200. 
The  ice  front  halted  and  built  the 
first  terminal  moraine  ( TM )  and  the 


FIG.  200. — Diagram  to  illustrate  the  forma- 
tion of  a  terminal  moraine  (TM),  a  re- 
cessional moraine  (RM),  outwash  plains 
(OWP),  and  valley  train. 


outwash  water  built  the  outwash 

plain  (OWP).     Then  the  ice  front 

melted  back  a  considerable  distance 

without  any  notable  halts  and  the  escaping  waters  built  up  the  valley  to  a  fairly  level 

plain,  the  valley  train.     Another  long  halt  resulted  in  another  moraine  (recessional 

moraine,  RM)  and  another  outwash  plain  (OWP) . 

Kames  and  Eskers 

Two  other  fluvio-glacial  forms  are  interesting,  although  not  impor- 
tant from  a  standpoint  of  soils.  Under  some  conditions,  heavily  laden 
glacial  streams  sometimes  built  hills  of  gravel  which  are  called  kames. 
Eskers  are  winding  ridges  of  sand  and  gravel  (Fig.  201) ;  they  are  often 
locally  called  "  serpent  ridges."  Their  materials  are  usually  irregularly 


218  GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 

stratified.  In  some  cases  they  are  several  miles  in  length,  although 
they  are  seldom  continuous  for  long  distances.  Eskers  are  evidently 
the  beds  of  streams  which  flowed  in  or  beneath  the  ice  and  when  the 
ice  melted  these  winding  stream  beds  were  left.  Both  kames  and  eskers 
make  coarse-textured,  droughty  and  infertile  soils,  usually  uncultivated, 


FIG.  201. — An  esker  in  Michigan.     It  curves  from  right  to  left.     (Russell,  U.  S. 

Geological  Survey.) 

but  in  some  cases  they  are  in  forest.     Locally  they  are  important 
sources  of  gravel  and  sand. 

Typical  Area. — Fig.  196,  which  shows  an  occurrence  of  ground  moraine,  terminal 
moraine  and  outwash  plain  together  with  some  other  glacial  features,  will  afford  a 
brief  review  of  glacial  work.  The  terminal  moraine  is  here  termed  the  "  kettle 
moraine  "  because  of  the  hundreds  of  kettle-like  depressions.  The  topography  is  - 
extremely  complex;  parallel  and  diverging  low  ridges,  depressions,  occasional  flat- 
topped  hills  and  hundreds  of  large  and  small  irregularly  shaped  hills  "  combine  to 
form  a  wilderness  of  humps  and  hollows  "  (Alden).  The  ground  moraine  has  a 
flat  to  undulating  surface  with  drainage  so  poor  that  much  of  the  area  is  swampy. 
Shallow  depressions  are  occupied  by  lakes,  many  of  which  have  no  outlets.  Other 
features  often  associated  with  the  ground  moraine  in  this  and  many  other  regions 
are  the  drumlins,  the  low  ovate  hills  which  are  seen  rising  above  the  surface  in  the 
northwestern  part  of  the  area.  The  outwash  plain  here  shows  the  usual  steeper 
slopes  near  the  terminal  moraine  with  very  gentle  slopes  farther  to  the  east. 

The  drift  in  this  region  is  gravelly  and  sandy  with  frequent  boulders  and  com- 
paratively little  clay.  As  a  result  the  soils  of  the  terminal  and  ground  moraines 
are,  as  a  whole,  coarse  textured.  Drainage  on  the  terminal  moraine,  both  surface 
and  subsurface,  is  excessive  because  of  the  sandy  and  gravelly  soils  and  much  of  this 
feature  is  non-agricultural  land.  On  the  other  hand,  while  the  soils  of  the  ground 
moraine  are  sandy,  the  level  surface  and  slow  drainage  have  produced  such  an 
accumulation  of  humus  and  other  vegetable  matter  that  the  soils  are  of  a  somewhat 


KAMES  AND  ESKERS 


219 


"  stiff  "  character;  the  poor  drainage  has  also  led  to  an  accumulation  of  peat  in 
places.  Many  of  the  patches  of  sandy  soils  seen  in  the  ground  moraine  are  due  to 
local  outwash  from  the  glacier  as  it  was  retreating.  The  soils  of  the  outwash  plain 
are  loams  for  the  most  part  with  beds  of  sand  and  gravel  often  underlying  the  sub- 
soil. The  drainage  is  good  and  these  soils  are  fairly  productive.  It  will,  of  course, 
be  noted  that  there  are  many  con- 
trasts between  the  soils  in  this 
area  and  those  shown  in  Fig.  199 
(Long  Island)  and  these  contrasts 
emphasize  the  fact  of  soil  varia- 
bility in  glacial  materials. 


TOPOGRAPHIC  AND  DRAINAGE 
CHANGES  DUE  TO  GLACIA- 
TION 


FIG.  202. — Diagram  to  illustrate  the  smoothing 
of  a  rough  preglacial  topography  (left)  and 
roughening  of  a  smooth  preglacial  topog- 
raphy (right). 

The  changes  due  to  glacia- 

tion  in  soil  composition  and  in  topography  and  drainage  have  been 
noted  to  some  extent.  Topography  and  drainage,  as  affected  by  glacia- 
tion,  deserve  further  consideration  because  they  are  very  important 

soil  factors.  Topographic  changes, 
due  to  glaciation,  include  both  de- 
structive and  constructive  changes, 
both  erosion  and  deposition.  A 
somewhat  hilly  region  may  be 
smoothed  both  by  erosion  and  by 
deposition,  while  a  level  region 
may  be  so  covered  by  forms  of 
drift  such  as  drumlins  and  terminal 
moraines,  for  instance,  as  to  have 
a  rougher  topography,  Fig.  202. 
An  area  in  southeastern  Wisconsin 
is  illustrative,  Fig.  203.  The  dia- 
gram shows  a  preglacial  area  with 
a  somewhat  rugged,  mature  topog- 
raphy which  was  completely  cov- 
ered with  drift  and  the  former 
topography  entirely  obliterated. 


FIG.  203. — An  area  in  Wisconsin.  Uppei 
diagram,  probable  preglacial  topogra- 
phy and  drainage;  lower  diagram, 
present  features.  (Data  after  Alden, 
U.  S.  Geological  Survey.) 


The  present  youthful  topography 
is  rather  flat  except  where  it  is 
crossed  by  a  terminal  moraine. 

The  present  Rock  River  flows  a  little  west  of  its  old  valley  while 
Turtle  Creek  flows  across  the    former  preglacial  divide. 


220  GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


FIG.  204. — Unglaciated  valley,  Utah.     (Atwood,  U.  S.  Geological  Survey.) 


r 


FIG.  205.— Glaciated  valley,  Utah.     (Atwood,  U.  S.  Geological  Survey.) 


FEATURES  OF  EROSION 


221 


Features  of  Erosion 

The  erosive  effects  of  ice  on  plains  and  other  somewhat  level  sur- 
faces appear  to  be  slight;  indeed  there  are  large  areas  where  the  con- 
tinental glaciers  failed  to  remove  even  the  soils,  for  buried  soils  are  often 


FIG.  206. — Distant  view  (upper)  and  close  view  (lower)  of  a  cirque,  Canada.     (Can- 
adian Geological  Survey.) 

found  beneath  later  glacial  deposits,  Fig.  214.  However,  when  the  flow 
of  thick  ice  is  concentrated  in  valleys,  there  is  often  great  erosion.  The 
valley  sides  are  smoothed  and  if  the  valley  is  V-shaped  it  is  likely  to  be 
worn  to  a  U-shaped  section,  Figs.  204  and  205.  Valleys  are  often  deep- 
ened so  that  tributary  valleys  are  left  above  the  main  valley  and  form 
the  hanging  valleys  so  often  found  in  glaciated  regions.  Fiords  are  gla- 
ciated valleys  near  a  coast  into  which  the  sea  has  been  allowed  to  enter 


222 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


by  a  sinking  of  the  coast;  they  are  thus  due  to  a  combination  of  coastal 
depression  and  to  glacial  erosion.  Cirques  are  ampitheater-like  open- 
ings often  found  at  the  heads  of  glaciated  mountain  valleys,  Fig.  206. 
These  erosion  features  are  most  characteristic  of  valley  glaciers  although 
they  are  by  no  means  confined  to  them. 

Continental  glaciers,  because  of  their  great  mass  of  ice  and  their 
longer  journeys,  have  more  opportunity  for  acquiring  loads  and  are  in 
consequence  more  important  as  depositing  agents  than  valley  glaciers. 
While  it  is  true  that  some  terminal  moraines  are  hilly,  yet  in  the  main 
the  topography  due  to  continental  glaciation  is  youthful  and  the  drain- 
age immature  and,  on  the  whole,  a  region  covered  with  drift  is  usually 
smoother  than  before  glaciation. 


Drainage  Changes 

These  changes  due  to  glaciation  are  mainly  caused  by  deposition,  for, 
while  ice  erosion  effected  minor  changes  in  drainage  lines,  the  main 
changes  have  been  due  to  the  blocking  of  old  drainage  lines  by  deposition 
of  drift.  In  many  preglacial  stream  valleys  the  drift  was  deposited 

in  places  so  as  to  turn  the  stream 
out  of  its  former  course,  and  in 
fact  entire  stream  valleys  were 
sometimes  filled  with  such  thick 
drift  deposits  that,  when  the  ice 
withdrew,  the  stream  was  forced 
to  take  an  entirely  new  course, 
Fig.  203.  Such  streams  are  termed 
diverted  streams.  A  fine  example 
is  seen  in  southeastern  Iowa  where 
the  Mississippi  River  flows  for  the 
most  part  in  a  broad  valley,  but 
near  Keokuk  the  valley  narrows 
for  about  25  miles  and  then  widens 
to  its  former  width,  Fig.  207;  a  few 
miles  west  the  old  river  valley  has 

been  filled  with  drift  which  so  completely  blocked  the  drainage  that  the 
Mississippi  was  compelled  to  cut  the  present  new  valley.  Such  features 
are  of  small  direct  agricultural  importance,  but  are  of  great  economic 
importance,  since  many  of  the  great  water-power  sites  like  those  at 
Keokuk,  Minneapolis  and  Rochester  are  due  to  diverted  streams.  River 


FIG.  207. — Preglacial  and  postglacial 
valleys  of  the  Mississippi  in  southeast- 
ern Iowa.  The  buried  preglacial  val- 
ley is  indicated  by  dotted  lines. 


MARGINAL  GLACIAL  LAKES  223 

valleys  have  even  been  so  filled  with  drift  that,  upon  the  departure  of 
the  ice,  the  stream  flow  is  reversed,  forming  reversed  streams.  Such  is 
the  upper  Allegheny  River,  where  oil  wells  sunk  along  this  valley  show 
the  rock  floor  to  be  sloping  northward  while  the  present  flow  of  the 
stream  is  southward. 


Marginal  Glacial  Lakes     . 

It  has  been  noted  that  when  the  ice  margin  was  on  plains  and  valleys 
sloping  away  from  the  ice,  the  water  from  the  melting  ice  flowed  away 
and  often  built  outwash  plains  and  valley  trains.  On  the  other  hand, 
when  the  land  surface  sloped  toward  the  ice  the  waters  tended  to  accumu- 
late between  the  ice  front  and  the  higher  land  in  front,  thus  forming 


FIG.  208. — Section  of  a  marginal  glacial  lake.     The  drainage  has  been  blocked  by 

ice  to  the  right. 

marginal  glacial  lakes.  This  is  illustrated  in  Fig.  208,  where  the  front 
of  the  glacier  has  melted  back  from  a  divide  and  a  marginal  lake  has 
formed  with  the  glacier  acting  as  a  dam  to  hold  the  water.  The  water 
accumulated  until  it  rose  to  the  lowest  point  in  the  land  barrier,  where  it 
found  an  outlet,  and  often  as  the  ice  melted  back  lower  outlets  were 
uncovered  so  that  many  marginal  lakes  had  several  outlets.  Finally, 
as  the  ice  melted  away,  the  lake  was  drained  and  became  extinct. 

Fig.  209  illustrates  the  changes  in  the  extinct  glacial  Lake  Maumee.  In  the 
upper  diagram  (A),  the  ice  has  melted  back  and  a  small  lake  has  accumulated  near 
the  margin,  draining  to  the  southwest;  the  hilly  belt  (terminal  moraine)  marks  a 
position  of  the  ice  margin  when  the  glacier  halted.  In  the  diagram  (B),  the  lake 
has  enlarged  and  discharges  to  the  northwest;  a  second  terminal  moraine  marks  a 
prolonged  halt  of  the  ice  margin. 

Many  of  the  outlets  of  these  lakes  were  of  short  duration,  but  some 
persisted  for  so  long  that  they  cut  deep  wide  channels  through  solid  rock. 
A  case  in  point  is  the  outlet  of  the  former  glacial  Lake  Chicago,  the 
predecessor  of  the  present  Lake  Michigan;  the  old  outlet  is  a  deep,  wide 


224 


GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 


valley  through  which  a  canal  has  been  cut  connecting  Lake  Michigan 

with  the  Illinois  River. 

Not  only  were  marginal  lakes 
formed  during  the  ice  retreat,  but 
doubtless  they  were  formed  during 
the  advance  of  the  glacier.  The 
traces  of  such  lakes  have,  in  most 
cases,  been  obliterated  by  the  ice 
which  advanced  over  their  sites, 
but  their  existence  has  been  inferred 
in  some  places  by  the  presence  of 
evident  lake  materials  in  moraines. 
These  old  lake  sediments  also  add 
to  the  complexity  of  the  drift. 

Features  of  Abandoned  Glacial 
Lakes. — Several  features  charac- 
terize these  abandoned  lakes. 
Shore  lines,  such  as  beaches,  beach 
ridges,  bars  and  deltas  are  in  places 
so  prominent  and  well  preserved 
as  to  be  evident  to  almost  any  one. 
Often  there  are  two  or  more  shore 
lines,  one  above  the  other,  due  to 
fluctuating  levels  of  the  water. 
The  old  outlets  are  often  obscure 
to  the  untrained  observer  but  many 
are  easily  recognized  as  the  work 
of  streams  that  have  vanished. 
Often,  during  an  ice  halt,  a  ter- 
minal moraine  was  built  in  the  lake 
and  sometimes  partly  covered  with 
lake  sediments.  The  lake  bottoms 
in  the  deeper  portions  are  almost 
invariably  very  level  plains  unless 
they  have  been  eroded  since  the 


FIG,  209. — Diagrams  showing  stages  in 
the  glacial  Lake  Maumee.  Present 
drainage  in  C.  (Data  after  Leverett 
and  Taylor,  U.  S.  Geological  Survey.) 


glacial  period,  Fig.  232. 

Soils. — Like    all    lakes,    these 

marginal    glacial    lakes    acted    as 

huge  settling  tanks  for  the  sediments  carried  to  them  by  the  streams,  the 
sand,  gravel  and  coarser  materials  being  deposited  near  shore  where 


MARGINAL  GLACIAL  LAKES 


225 


streams  enter  and  the  finer  materials  were  carried  into  deeper  water 
farther  from  shore.  However,  there  is  an  important  distinction  between 
these  glacial  lakes  and  other  lakes;  the  debris  from  the  melting  ice  front 
is  added  to  the  stream-carried  sediments  so  that  marginal  lake  soils  may 
contain  much  till  material.  Another  contrast  is  that,  whereas  many 
lake  sediments  are  composed  of  well-weathered  materials,  the  sediments 
of  marginal  glacial  lakes  contain  much  fresh  material  ground  up  by  the 
glaciers  and  this  is  one  cause  of  the  usual  fertility  of  these  soils.  Again 
many  soils  from  lakes  of  this  class  show  much  less  leaching  and  oxidation 
than  soils  of  similar  materials  surrounding 
the  lakes,  a  feature  obviously  due  in  large 
measure  to  the  protective  covering  of  the 
former  lake  waters. 

Another  very  common  characteristic 
of  marginal  glacial  lakes  is  their  more 
or  less  complex  shore  lines.  As  the  glacier 
front  advanced  or  retreated  and  closed  or 
opened  new  outlets,  the  surface  of  the 
lake  rose  and  fell  and  made  new  shore 
lines  at  each  level  provided  the  level  was 
maintained  for  a  considerable  time.  Fig. 
210  shows  the  changes  in  level  of  an 
extinct  lake.  The  outer  line  represents 
the  lake  at  its  highest  stage.  From  this 
level  the  old  lake  surface  with  many 
pauses  and  readvances  fell  to  lower  levels. 
As  a  consequence  of  these  changing  shore 
lines,  there  is  a  belt  of  sandy  soils  near 

the  margins  of  the  lakes  because  the  waves  and  currents  here  swept 
away  the  finer  materials. 

An  illustration  of  this  is  seen  in  Fig.  211  which  shows  the  soils  and  other  features 
in  a  small  area  formerly  covered  by  the  extinct  glacial  Lake  Agassiz.  The  waves 
and  currents  acting  at  different  levels  formed  four  distinct  ridges  in  the  western 
part  of  this  area  giving  a  belt  of  sandy  soils  about  10  miles  wide.  The  sandy  soils 
are  due  entirely  to  the  sorting  by  the  water  and  not  to  the  till  and  lake  sediments 
on  which  the  waves  worked.  East  of  the  sandy  belt  is  the  level  lake  bottom  on 
which  are  heavy  clay  loams.  Such  extinct  glacial  lakes  varied  in  size  from  a  few 
square  miles  to  hundreds  of  square  miles  in  extent.  They  are  of  especial  agricul- 
tural interest  because  of  the  large  areas  of  exposed  lake  bottoms  which,  on  the 
whole,  yield  very  productive  soils  and  their  importance  warrants  further  consider- 
ation of  some  typical  examples 


FIG.  210.— Some  of  the  shore 
lines  of  Lake  Agassiz.  (After 
Upham,  U.  S.  Geological 
Survey.) 


226 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


The  glacial  Lake  Agassiz  is  a  relatively  simple  type,  Fig.  212.     At 
its  maximum  extent  it  was  about  700  miles  long,  250  miles  wide  and 


FIG.  211. — Part  of  the  lake  bed  and  shores  of  the  former  Lake  Agassiz  in  North 
Dakota.     The  dotted  lines  indicate  the  distinct  soils  of  the  shore  lines. 

covered  an  area  of  about  110,000  square  miles,  an  area  about  twice  as 
large  as  New  England  or  considerably  larger  than  the  present  Great 

Lakes.  As  the  ice  front  melted 
northward  over  the  watershed 
between  the  Minnesota  River 
and  the  Red  River  of  the  North, 
the  water  accumulated  and  for 
a  long  time  flowed  into  the 
Minnesota  River.  Later  an 
outlet  farther  north  was  un- 
covered, finally  the  ice  disap- 
peared and  the  present  drainage 
into  Hudson  Bay  was  estab- 
lished. The  old  lake  bottom 

FIG.  212.-Map  showing  the  greatest  extent  is  now  a  very  level  kke  Plain 
of  the  glacial  Lake  Agassiz.  The  present  except  where  recessional  mo- 
Lake  Winnipeg  occupies  a  small  part  of  raines  were  built  in  the  lake, 
the  extinct  lake  bottom.  (After  Upham,  and  even  these  for  the  most 
U.  S.  Geological  Survey.)  part  loge  ^  rough  knobg  and 

hills  of  the  land  phase,  and  be- 
come belts  of  slightly  undulating  surface.  Lake  Winnipeg  and  many 
other  lakes  occupy  depressions  in  the  lake  plain. 

The  Great  Lake  region  was  occupied  by  a  complex  series  of  marginal 
glacial  lakes  of  which  the  present  lakes  are  the  successors.    Little  is 


MARGINAL  GLACIAL  LAKES 


227 


i 


known  of  the  preglacial  topography  of  this  region  except  that  it  was 
a  lowland  extending  from  New  York 
to  Wisconsin.  The  glaciers  first  ad- 
vanced entirely  over  this  region,  later 
they  retreated  with  many  halts;  they 
extended  across  the  present  St.  Law- 
rence drainage  lines,  blocking  the 
drainage  and  forming  a  series  of  lakes 
which  outflowed  to  the  southward. 
In  addition  there  was  some  tilting  of 
the  land  which,  combined  with  the 
various  ice  advances  and  retreats, 
added  to  the  complexity  of  these 
marginal  lakes.  Some  of  the  differ- 
ent lake  stages  are  shown  in  Fig. 
213,  which  should  be  kept  in  mind 
as  the  description  follows. 

The  glacial  Lake  Chicago  occu- 
pied the  Lake  Michigan  Basin  and 
outflowed  into  the  Illinois  River; 
Lake  Duluth  occupied  the  western 
part  of  the  Lake  Superior  Basin  and 
found  its  outlet  along  the  present 
St.  Croix  River;  Lake  Maumee  oc- 
cupied the  western  part  of  the  Lake 
Erie  Basin  and  outflowed  into  the 
Wabash  River.  Many  later  changes 
occurred,  some  of  which  are  shown 
in  the  maps  of  Fig.  231.  Lake  Mau- 
mee, which  occupied  the  western 
part  of  the  Lake  Erie  Basin,  may  be 
taken  as  an  example  of  these  lakes, 
Fig.  209.  At  first  a  small  lake  was 
formed  which  flowed  past  Fort 
Wayne,  Ind.,  into  the  Wabash  River. 
As  the  ice  retreated  northward  the 
lake  increased  in  size  and  later  the 
glacier  receded  and  exposed  a  lower 
outlet  through  Michigan  into  the 
Lake  Michigan  Basin,  when  the  water  deserted  the  Fort  Wayne  outlet. 


FIG.  213. — Different  stages  of  the 
great  glacial  lakes  (dotted  areas). 
Outlets  are  shown  by  heavy  lines, 
with  arrows.  (After  Leverett  and 
Taylor,  U.  S.  Geological  Survey.) 

.The  upper  maps  show  earlier  stages.  The 
question  marks  indicate  that  knowledge 
of  the  margin  is  at  present  incomplete. 


228 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


Ihe  bed  of  Lake  Maumee  in  northwestern  Ohio  is  a  level  plain  broken 
here  and  there  by  low,  inconspicuous  ridges  of  terminal  moraines  which 
were  laid  down  in  the  lake.  Old  shores  are  generally  marked  by  beach 
ridges  which  rise  above  the  surface  and  the  sandy  soils  of  these  ridges 
are  often  in  sharp  contrast  with  the  heavy  soils  of  the  adjoining  lake 
plain. 


STAGES  IN  THE  GLACIAL  PERIOD 


40  FT. 


30  FT.  _ 


20  FT.  _ 


BLUE  TILL   (WISCONSIN) 


Introductory.  —  We  have  assumed,  for  convenience  of  discussion,  that 
the  great  continental  glaciers  advanced  to  their  southern  limit,  then 
retreated  and  finally  disappeared,  but  careful  studies  have  shown  that 

there  were  a  series  of  glacial  in- 
vasions (stages)  each  separated 
by  intervals  of  no  glaciation 
(inter  glacial  stages).  In  proof 
of  this,  one  traveling  from  St. 
Louis  to  Chicago  will  pass  over 
at  least  three  different  drifts; 
the  drift  in  southern  Illinois  is 
different  in  texture,  composition 
and  topography  from  that  in 
the  northern  part  of  the  state. 
A  deep  boring  through  the  drift 
in  northern  Illinois  and  in  many 
other  places  in  the  Middle  West 
will  often  show  a  succession  of 
drifts,  buried  soils  and  buried 
swamps  as  shown  in  Fig.  214. 
A  study  of  this  section  shows 
that  first  the  glacier  deposited 


10FT._ 


BROWN  CALCAREOUS 
"LOESS  (WISCONSIN) 


BROWNISH  PEATY  SILT 
CONTAINING  A  LARGE 

"AMOUNT  OF  WOOD 
(SANGAMON) 

DARK  SILT  WITH 
"FIBROUS  ROOTS 


REDDISH  BROWN  LEACHED 
"TILL  (ILLINOIAN) 


LEACHID  TILL 


the  lower  till  (Illinoian)  and  then 
retreated  this  till  was  exposed 
to  weathering  for  a  long  time,  a 
time  so  long  that  the  upper 
portion  of  the  till  became 
leached  and  a  thick  layer  of  peat 

and  soil  was  formed.     After  this  the  winds  blew  a  layer  of  dust  (loess) 
over  the  surface.     Then  the  glacier  again  advanced,   burying  these 


FIG.  214 — A  section  of  drift  in  Illinois  show- 
ing the  Wisconsin  and  Illinoian  drifts 
separated  by  soils  of  the  Sangamon  inter- 
glacial  period.  (After  Levefett,  U.  S. 
Geological  Survey.) 


STAGES  IN  THE  GLACIAL  PERIOD 


229 


materials  beneath  a  deep  layer  of  later  (Wisconsin)  drift.  These  and 
many  other  observations  show  that  the  glacial  period  was  not  a  simple 
advance  and  recession  of  the  ice,  but  rather  a  succession  of  advances 
alternating  with  extensive  recessions  during  which  the  evidences  point 
to  a  relatively  mild  climate. 

In  many  cases  the  advancing  ice  eroded  the  drift  of  the  preceding 
stage,  but  we  have  seen  that  glaciers  often  ride  over  loose  debris  without 
greatly  disturbing  it  so  that  in  many  places  borings  show  two  or  more 
different  drifts,  one  above  the  other  and  often  separated  by  buried  soils, 
Fig.  214.  From  the  standpoint  of  soils  the  distribution  of  drifts  is 
very  important.  It  will  be  seen  in  Fig.  215  that  in  general  the  older 
ice  advances  and  their  drifts  (Kansas  and  Illinoian)  extend  farther  south 
so  that  from  south  to  north  there  are 
successively  younger  drifts  exposed. 
In  other  words,  with  one  probable 
exception  the  older  invasions  ex- 
tended farthest  south  and  each  suc- 
ceeding advance  extended  to  less 
and  less  distances  southward.  It 
is  obvious  that  the  older  drifts 
influence  soils  only  when  they  lie 
beyond  the  overlying  younger  drifts 
or  when  the  latter  have  been  eroded, 
especially  in  valleys  where  the  older, 
underlying  drifts  may  influence  the 

soils  of  the  valley  bottom  and  sides.  Furthermore,  it  must  not  be 
assumed  that  the  old  drifts  extend  continuously  beneath  the  younger 
drifts.  In  fact,  all  the  drifts  have  never  been  found  in  any  one  -place 
overlying  each  other  and  in  large  areas  underlying  drifts  are  entirely 
lacking,  especially  where  later  ice  erosion  was  vigorous. 

The  succession  of  older  drifts  extending  from  beneath  younger 
drifts  is  best  shown  in  the  upper  Mississippi  Basin  and,  in  fact,  this  is 
the  only  locality  in  North  America  where  there  are  large  areas  of  older 
drifts.  Studies  have  shown  the  presence  of  older  drifts  east  of  the 
Ohio  but,  for  the  most  part,  the  exposures  are  scattering  so  that,  from 
a  standpoint  of  soils,  the  older  drifts  here  are  almost  negligible. 

Soils  and  Glacial  Stages.— From  an  agricultural  point  of  view,  there 
are  several  important  differences  in  these  drifts.  (1)  Perhaps  one  of 
the  most  important  contrasts  is  the  greater  degree  of  weathering  to 
which  the  older  drifts  have  been  subjected.  The  older  drifts  have  been 


WISCONSIN  DRIFT  V.V.V  KANSAN  DRIFT   -^EZrEE: 
IOWAN.  DRIFT  -f-  -f-  +      ILLINOIAN  DRIFT 


FIG.  215. — Map  showing  exposures 
of  different  glacial  drifts.  (After 
Leverett  and  Calvin.) 


230 


GLACIERS  AND  GLACIATION:  GLACIAL  SOILS 


oxidized  to  considerable  depths  with  the  result  that  the  soils  ordinarily 
have  reddish  or  yellowish  colors  which,  at  variable  depths,  change  to 

the  ordinary  bluish  or  grayish 
colors  of  the  till.  (2)  The 
boulders  and  rocks  of  the 
older  till  are  naturally  more 
weathered;  often  the  granitic 
rocks  in  old  till  are  so  weath- 
ered that  they  can  be  crum- 
pled by  the  fingers.  (3)  Older 
exposed  drifts  are  much  more 
leached  and  their  soluble 
materials  carried  away. 

The  soils  of  Illinois,  which  are 
mostly  glacial,  furnish  an  interest- 
ing illustration  of  leaching  in 
different  drifts  which  are  shown 
in  Fig.  216.  The  ice  here  re- 
treated in  a  general  northerly  di- 
rection, exposing  in  succession 
several  different  drifts,  the  south- 
ernmost being  oldest  and  most 
widely  exposed.  The  average  an- 
alyses of  these  different  soils  according  to  Hopkins  and  Petit  are  as  follows:1 


FIG.  216. — Map  of  Illinois  showing  different 
drifts  as  follows  (after  Leverett):  (1)  un- 
glaciated;  (2)  lower  Illinoian;  (3)  middle 
Illinoian;  (4)  upper  Illinoian;  (5)  lowan 
and  pre-Illinoian ;  (6)  early  Wisconsin;  (7) 
late  Wisconsin.  Map  on  the  right  shows 
average  values  per  acre,  1909.  (U.  S. 
Census.) 


AVERAGE  NUMBER  OF  POUNDS  PER  ACRE  IN  SURFACE  SOIL  (0-7  INCHES)  AND  IN 
LOWER  SUBSOIL  (20-40  INCHES). 


DRIFT. 

SOIL. 

SUBSOIL. 

Total 
phos- 
phorus. 

Total 
potash. 

Total 
phos- 
phorus. 

.  Total 
potash. 

Early  Wisconsin,  No.  7.       

1,360 
1,360 
1,240 
1,140 
900 

34,700 
43,230 
32,490 
32,520 
28,390 

2,970 
2,980 
3,230 
2,840 
2,780 

110,200 
147,410 

98,580 
94,640 
91,980 

Late  Wisconsin,  No.  6  

Upper  Illinoian,  No.  4  

Middle  Illinoian,  No.  3. 

Lower  Illinoian  No  2 

It  is  evident  that,  in  general,  the  older  drifts  contain  less  phosphorus  and  potash 
than  the  younger  drifts  and  this  difference  is  doubtless  due  in  large  measure  to  the 
longer  weathering  and  leaching  of  the  older  exposed  drifts.     The  composition  is 
i  Bulletin  No.  123,  Illinois  Agricultural  Experiment  Station,  1908. 


STAGES  IN  THE   GLACIAL  PERIOD  231 

not  entirely   consistent  with  the  age  of  the  drifts,  but  this  is  to  be  expected,  since 
some  of  the  drifts  come  from  different  sources  and,  moreover,  all  drift  is  variable. 

(4)  Finally  the  younger  drifts  are,  as  a  rule,  less  eroded  than  the 
older,  Fig.  217.  It  has  been  repeatedly  noted  that  the  humus  content 
of  a  soil  is  to  a  considerable  extent  dependent  on  the  drainage  and  the 
drainage  in  turn  is  closely  related  to  the  topography;  as  a  result,  the 
soils  on  newer  drift  ordinarily 
contain  more  humus.  Some 
soils  of  the  very  latest  drift  are 
so  recent  there  is  comparatively 
little  difference  between  soil 
and  subsoil  because  there  has 
not  been  time  for  the  soil  FlG.  217.-Diagram  to  illustrate  the  topog- 
water  to  carry  down  the  finer  raphy  and  drainage  on  new  drift  (A\  old 
silts  and  clays  into  the  subsoil.  drift  (/?),  and  an  unglaciated  area  (C). 
It  must  be  remembered  that 

many  of   the    older   drifts   are  covered  with  loess,  which  constitutes 
the  soil  material  and  so  lessens  the  influence  of  the  underlying  drifts. 

Other  features  of  less  agricultural  interest  but  of  much  scientific 
interest  are  (1)  buried  soils,  peat  bogs,  lake  beds  and  land  fossils  some- 
times found  between  drifts;  (2)  interglacial  valleys  which  were  cut 
between  ice  advances,  buried  by  later  invasions  and  occasionally  dis- 
closed by  stream  cutting  or  well  borings  and  (3)  there  are  naturally 
found  differences  in  rocks  and  till  and  in  size  of  materials.  In  some 
cases  no  single  one  of  the  foregoing  criteria  would  prove  the  existence 
of  different  drifts,  but  the  association  of  several  criteria  is  convincing. 

Stages  in  the  Glacial  Period. — Six  stages  and  five  interglacial  stages 
in  the  Mississippi  Basin  have  been  recognized  as  follows:1 

Late  Wisconsin  Stage. 

Early  Wisconsin  Stage. 

Peorian  Interglacial  Stage. 

lowan  Stage. 

Sangamon  Interglacial  Stage. 

Illinoian  Stage. 

Yarmouth  Interglacial  Stage. 

Kansan  Stage. 

Aftonian  Interglacial  Stage. 

Sub-aftonian  Stage. 

1  Abridged  from  Chamberlin  and  Salisbury,  Geology,  Vol.  3,  1904. 


232 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


The  Sub-aftonian  stage  is  typically  found  in  Iowa,  where  it  underlies 
younger  drift  and  is  not  much  exposed  except  in  valleys.  The  till,  as 
would  be  expected,  is  well  weathered  and  contains  a  high  percentage 
of  igneous  rocks;  it  also  contains  some  sand  and  gravel  which  are 
sources  of  water  supply  for  wells. 

The  Aftonian  Interglacial  Stage  is  marked  by  long  weathering;  it 
contains  much  gravel  and  sand  and  is  also  characterized  by  buried 
peat  and  trees. 

The  Kansan  Stage  is  represented  by  widespread  drift,  Fig.  215, 
covering  extensive  areas  in  Kansas,  Missouri,  Iowa  and  Nebraska. 


FIG.  218. — Topography  on  Kansan  drift  in  Iowa.     (Iowa  Geological  Survey.; 

The  till  has  a  high  clay  content  and,  except  locally,  a  low  content  of 
boulders.  This  drift  is  noteworthy  for  its  very  low  content  of  water- 
laid  materials;  stratified  gravel  and  sand  are  rather  rare  in  this  drift 
as  a  whole.  Their  absence,  which  indicates  but  slight  water  from  melting 
ice,  has  not  yet  been  adequately  explained.  Perhaps  the  ice  disap- 
peared by  evaporation  more  than  by  melting.  Moraines,  kames,  eskers 
and  other  features  common  in  later  drift  are  almost  if  not  quite  absent. 
The  soils  from  this  drift  are  somewhat  heavy  as  a  rule  and  silt  loams 
are  common  types. 

The  Yarmouth  Interglacial  Stage  was  of  considerable  duration,  as  is 
shown  by  the  deeply  eroded  exposed  Kansan  drift  and  by  its  weathered 
condition.  This  was  followed  by  the  Illinoian  Stage,  which  has  its 
greatest  exposure  in  Illinois  and  adjacent  states,  Fig.  215.  The  till 


STAGES  IN  THE  GLACIAL  PERIOD  233 

is  high  in  clay  and  usually  not  stony.  The  upper  portions  are  leached 
of  their  soluble  lime  carbonate  for  a  short  distance  below  the  surface 
and  are  of  yellowish-brown  colors  which  grade  into  gray  or  bluish-gray 
below.  Like  the  Kansan  drift,  there  is  little  outwash  or  sorted  material, 
although  there  is  more  than  in  the  Kansan  drift.  There  are  compara- 
tively few  moraines,  eskers  and  kames  as  compared  with  later  drifts, 
The  original  surface  was  somewhat  level  although  it  is  now  consid- 
erably eroded  in  places,  but  the  erosion  has  naturally  not  been  so  great 
as  in  the  Kansan  stage.  Widespread  loess  deposits  cover  much  of  this 
drift  so  that  the  direct  influence  of  Illinoian  drift  on  soils  is  not  so 
great  as  in  many  other  drifts.  The  Illinoian  stage  was  followed  by  the 
Sangamon  Interglacial  Stage,  which  was  of  shorter  duration  than  the 
Yarmouth  stage. 


FIG.  219. — lowan  topography  in  Iowa.     (Iowa  Geological  Survey. 

The  lowan  Stage,  which  followed,  has  the  smallest  surface  exposure 
of  any  of  the  later  drifts.  Recent  studies  seem  to  show  that  this  drift 
should  be  included  with  the  Illinoian  drift,  but  the  question  is  not  yet 
settled.  This  drift  is  thinner,  is  somewhat  more  sandy  and  contains 
more  boulders  than  the  preceding  drifts;  like  them  it  contains  but  little 
water  deposited  material.  Closely  associated  with  this  drift  are  wide- 
spread deposits  of  loess  which  form  most  of  the  soils.  The  Peorian 
Interglacial  Stage  separates  this  stage  from  the  next  following  stage. 

The  Early  Wisconsin  and  Late  Wisconsin  stages  were  separated  by 
a  short  interglacial  period.  The  Wisconsin  drift  is  of  especial  soil 
interest  because,  while  not  extending  far  southward,  this  drift  is  of 
wider  exposure  since  it  was  not  covered  by  later  drifts  and,  furthermore, 


234  GLACIERS  AND  GLACIATION;  GLACIAL  SOILS 

the  Wisconsin  drift  is  not  much  covered  with  loess  as  compared 
with  earlier  drifts  so  that  the  soils  are  for  the  part  derived  directly  from 
the  drift.  This  drift  is  probably  the  most  studied  and  best  understood 
of  any  of  the  various  drifts,  in  part  because  it  is  the  most  recent  and, 
therefore,  best  shows  the  various  glacial  features.  Much  of  the  glacia- 
tion  in  the  western  mountains  of  North  America  was  accomplished 
during  the  Wisconsin  stages,  and  it  was  during  these  stages  that  the 
great  marginal  glacial  lakes  which  have  been  noted  were  formed. 

REFERENCES 
Stages  and  their  Drifts 

CHAMBERLIN  and  SALISBURY,  Geology,  Vol.  3,  1907,  pages  382-412. 
FRANK  LEVERETT  and  F.  B.  TAYLOR,  Monograph  53,  U.  S.  Geological  Survey,  1915, 
pages  10-32. 

THE  LOESS  AND  GLACIATION 

The  loess  has  been  considered  in  some  detail  under  a  different  head- 
ing but  it  should  here  be  noted  that,  both  in  North  America  and  Europe, 
important  areas  of  loess  are  associated  with  glacial  drift,  although  not 
all  areas  of  loess  are  so  associated  and  some  drifts  are  not  associated 
with  loess.  It  is  believed  that  the  freshly  exposed  drift  furnished  much 
of  the  fine  dust  which  now  constitutes  loess,  especially  in  North  America 
and  Europe.  In  North  America  the  largest  loess  areas  overlie  much  of 
the  Kansas,  Illinoian  and  lowan  drifts  and  even  some  of  the  eaily  Wis- 
consin areas  and,  moreover,  the  loess  not  only  overlies  these  drifts  but 
it  extends  in  many  places  far  beyond  the  drift  borders.  It  is  scarcely 
necessary  to  restate  the  fact  that  loess  makes  a  productive  soil ;  in  many 
places,  it  is  thick  enough  to  form  both  soil  and  subsoil  and  in  other 
places  it  forms  the  soil  while  the  underlying  materials  constitute  the 
subsoils. 

REFERENCE 

CHAMBERLIN  and  SALISBURY,  Geology,  Vol.  3,  1907,  pages  405-412  (The  Loess  in 
Connection  with  Glaciation). 

VALUE  OF  GLACIATION 

It  is  an  interesting  though  complex  question  as  to  whether  glaciation 
increased  or  decreased  the  soil  values  in  glaciated  districts.  Unques- 
tionably the  average  per  capita  wealth,  the  variety  and  extent  of  man- 
ufactures, the  commercial  development  and  in  many  places  the  average 
per  acre  value  of  agricultural  lands  are  greater  in  the  glaciated  districts 


STAGES  IN  THE  GLACIAL  PERIOD  235 

both  of  North  America  and  Europe,  but  it  would  be  unsafe  to  attribute 
these  facts  solely  to  glaciation.  Soil  values  themselves  are  affected  by 
commercial,  economic  and  sociological  factors  as  well  as  by  topography 
drainage,  texture,  composition  and  climate. 

On  the  whole,  glaciation  has  produced  good  soils.  They  are  less 
leached  and  the  supply  of  available  mineral  plant  food  is  greater  than 
in  most  residual  soils,  but  there  are  many  exceptions  in  the  case  of 
sandy  and  gravelly  fluvio-glacial  deposits.  The  general  effect  of  gla- 
ciation, especially  of  the  continental  type,  is  to  reduce  slopes  both  by 
scouring  off  projecting  elevations  and  especially  by  filling  depressions. 
Gentler  slopes  make  for  less  erosion  and  thicker  soils.  Furthermore, 
the  flat  or  gently  rolling  surfaces  of  ground  moraines  and  some  outwash 
plains  favor  the  accumulation  of  humus,  an  important  element  of  soil 
fertility.  The  soils  of. marginal  glacial  lakes  are  very  productive  and 
many  very  productive  loessial  soils  are  more  or  less  derived  from  gla- 
ciation. On  the  other  hand  the  extensive  level  surfaces  due  to  gla- 
ciation are  often  swampy  and  glacial  lakes  occupy  large  areas  of  other- 
wise available  lands.  Both  of  these  conditions  result  in  much  waste 
land  and  necessitate  extensive  use  of  ditch  and  tile  drainage. 

A  fair  comparison  of  the  effect  of  glaciation  on  land  values  can  be 
made  only  when  glaciated  and  unglaciated  lands  are  adjacent  and  as 
far  as  possible  alike  in  other  respects.  After  careful  study  of  soil 
and  crop  values  in  the  glaciated  and  unglaciated  areas  of  Wisconsin, 
Whitbeck  estimates  that  the  increased  valuation  of  glaciation  to  agri- 
culture amounts  to  about  $30,000,000  annually  in  spite  of  the  fact  that 
much  of  the  driftless  area  is  covered  with  fertile  loess.1  He  summarizes 
in  part  as  follows:  "  Notwithstanding  the  swamps  and  lakes  in  the 
glaciated  area,  61  per  cent  is  improved  farm  land  against  43.5  per  cent 
in  the  driftless  area.  In  the  fifteen  driftless  counties,  the  average  value 
of  farm  lands  and  farm  buildings  is  about  $12,000,000  per  county,  against 
$18,000,000  in  the  glaciated  area,  a  difference  in  favor  of  the  latter  of 
50  per  cent.  In  the  driftless  area,  there  are  on  the  average  over  126,000 
acres  per  county  of  woodland  and  woodland  pasture  against  about 
50,000  acres  in  the  glaciated  area,  the  difference  being  largely  due  to 
the  more  rugged  topography  of  much  of  the  driftless  area."  The  same 
general  trend  of  values  is  brought  out  by  Coffey's  land  value  map  of 
Ohio,  Fig.  220.2  The  soils  of  Illinois,  Fig.  216,  also  show  not  only  higher 

1  Op.  cit. 

2  Reconnoissance  Soil  Survey  of  Ohio,  George  N.  Coffey,  U.  S.  Bureau  of  Soils, 
1912. 


236 


GLACIERS  AND  GLACIATION;   GLACIAL  SOILS 


values  of  glaciated  soils  but  also  that,  in  general,  soils  of  newer  drifts  are 

more  valuable  than  those  of  older  drifts. 
However,  the  comparison  must  not  be 
carried  too  far,  for  we  do  not  know  the 
preglacial  soils.  On  the  other  hand,  the 
soils  of  New  England  have  probably 
lost  in  productiveness  through  glacia- 
tion.  The  glaciers  swept  away  the 
preglacial  soils  of  New  England  and, 
in  the  main,  deposited  stony  and  sandy 
materials  and  these  are  thin  in  many 
places.  It  is  very  suggestive  to  note 
that  much  the  same  rocks  and  topog- 
raphy in  regions,  of  New  Jersey,  Penn- 
FIG.  220.— Map  of  Ohio  showing  syivania  and  Virginia  have  productive 
land  values  in  dollars  per  acre  in  M  -,  .,  •  ,  ,  ,  ,, 

1909.  The  dotted  area  is  glaciated.  solls  and  *>  1S  probable  that  the  pre- 
(After  Coffey,  Ohio  Experiment  glacial  soils  of  New  England  were 
Station.)  equally  productive. 

CAUSES  OF  THE  GLACIAL  PERIOD 

The  causes  of  glacial  periods  are,  of  course,  climatic,  and  the  study 
of  living  glaciers  or  of  the  deposits  of  extinct  glaciers  gives  but  little 
clue  to  the  causes  of  the  last  glacial  period.  The  last  widespread  gla- 
ciation,  known  as  the  Glacial  Period,  is  not  the  only  one  in  the  earth's 
history,  for  there  are  indisputable  evidences  of  at  least  three  former 
periods,  one  of  which,  the  Permian  (see  page  296)  was  widespread. 
The  various  explanations  so  far  offered  have  failed  of  general  acceptance. 

REFERENCES 
General 

W.  C.  ALDEN,  The  Delaven  Lobe  of  the  Lake  Michigan  Glacier  of  the  Wisconsin 
Stage  of  Glaciation  and  Associated  Phenomena,  Professional  Paper  34,  U.  S. 
Geological  Survey,  1904. 

The  Dramlins  of  Southeastern  Wisconsin,  Bull.  273,  U.  S.  Geological  Survey,  1905. 

CHAMBERLIN  and  SALISBURY,  Geology,  Vol.  1,  1904;  General,  pages  232-268;  Trans- 
portation and  Erosion,  pages  275-284;  Deposition,  pages  284-290;  Glacio- 
fluvial  Work,  pages  290-293,  Vol.  3,  1907;  General,  pages  327-382. 

J.  GEIKIE,  The  Great  Ice  Age,  3d  Edition,  London,  1894. 

Earth  Sculpture,  Putnam,  1898,  Chapters  10-11  (Glaciation). 

W.  H.  HOBBS,  Earth  Features  and  Their  Meaning,  Macmillan,  1912;  General, 
pages  261-319;  Glacial  Lakes,  pages  320-339;  Land  Sculpture  by  Mountain 
Glaciers,  pages  367-389. 


STAGES  IN  GLACIAL  PERIOD  237 

FRANK  LEVERETT  and  F.  B.  TAYLOR,  Monograph  53,  U.  S.  Geological  Survey,  The 
Pleistocene  of  Indiana  and  Michigan. 

FRANK  LEVERETT,  The  Illinois  Glacial  Lobe,  Monograph  38,  U.  S.  Geological  Sur- 
vey, 1899. 

F.  E.  MATTHES,  Glacial  Sculpture  of  the  Bighorn  Mountains,  21st  Ann.  Kept.,  Part 
2,  U.  S.  Geological  Survey,  1900,  pages  167-190. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  Chapter 
on  Glacial  Deposits. 

I.  C.  RUSSELL,  Glaciers  of  North  America,  Ginn,  Boston,  1897. 

R.  D.  SALISBURY,  Physiography,  Holt,  1907:  General,  pages  207-241;  Transporta- 
tion and  Erosion,  pages  242-255;  Deposition,  pages  255-265;  Fluvioglacial 
work,  pages  265-270;  Work  of  Continental  Glaciers,  pages  270-274;  Drain- 
age Modifications,  pages  280-289. 

Glacial  Geology  of  New  Jersey,  N.  J.  Geological  Survey,  Vol.  5,  1902. 

TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapters  8-9. 

The  Great  Glacial  Lakes 

FRANK  LEVERETT  and  F.  B.  TAYLOR,  Monograph  53,  U.  S.  Geological  Survey, 
pages  316-519. 

RUSSELL  and  LEVERETT,  Ann  Arbor  Folio,  U.  S.  Geological  Survey,  1908. 

WARREN  UPHAM,  The  Glacial  Lake  Agassiz,  Monograph  25,  U.  S.  Geological  Survey, 
1896,  pages  192-274  (History). 

C.  F.  MARBUT  and  J.  E.  LAPHAM,  Soils  of  the  Glacial  Lake  and  River  Terrace  Prov- 
ince, Bull.  96,  U.  S.  Bureau  of  Soils,  1913;  General,  pages  165-169,  Soil  Series, 
pages  169-219. 

Glacial  Soils 

EDWARD  BARRETT,  Soils  of  Indiana,  38th  Ann.  Rept.  Ind.  Geological  Survey,  1914. 
C.  G.  HOPKINS  and  J.  H.  PETTOT,  Bull.  123,  111.  Agricultural  Experiment  Station, 

1908. 

FRANK  LEVERETT,  Surface  Geology  of  the  Southern  Peninsula  of  Michigan,  Pub- 
lication 9,  Mich.  Geological  and  Biological  Survey,  1912.    - . 
Surface  Formations  and  Agricultural  Conditions  of  Northwestern  Minnesota,  Bull. 

12,  Minn.  Geological  Survey,  1915. 
C.  F.  MARBUT  and  J.  E.  LAPHAM,  Soils  of  the  Glacial  and  Loessial  Province  in  Soils 

of  the  United  States,  Bull.  96,  U.  S.  Bureau  of  Soils,  1913;  General,  pages  109- 

116;  Soil  Series,  pages  116-164. 
New  Jersey  Geological  Survey,  Mechanical  and  Chemical  Composition  of  the  Soils 

of  the  Sussex  Area  by  A.  W.  Blair  and  Henry  Jenning,  Bull.  10,  1913. 
N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Report,  Part  1,  U.  S.  Geological 

Survey,  Glacial  Soils,  pages  236-39. 
U.  S.  Bureau  of  Soils,  Reconnoissance  Soil  Survey  of  Ohio,  1912;    Northwestern 

Pennsylvania,  1911;    Northeastern  Pennsylvania,  1908;    Puget  Sound    Basin, 

Eastern  Part,  Washington,  1909;    Puget  Sound  Basin,  Western  Part,  1910; 

Central-northern  Wisconsin,  1914;  Northeastern  Wisconsin,  1913. 
SAMUEL  WEIDMAN,  Soils  and  Agricultural  Conditions  in  North  Central  Wisconsin, 

Wisconsin  Geological  and  Natural  History  Survey,  Bull.  11,  1903. 
Reconnoissance  Soil  Survey  of  Northwestern  Wisconsin,  Wisconsin  Geological  and 

Natural  History  Survey,  Bull.  23,  1911. 


CHAPTER  XI 

r^;AV^-^.'~-;  £•«  '  i' 

LAKES  AND  SWAMPS;    LACUSTRINE  AND  CUMULOSE  SOILS 5 
LAKES;    LACUSTRINE  SOILS 

«  ->>  .'»«  •»  f-^>     »..»-      ;•»_..*•'  ••'•     ••'£•••      ' 

Introductory. — Lakes  and  swamps  are  often  closely  associated  and 
grade  into  each  other  so  that  it  is  appropriate  to  consider  them  together. 
A  shallow  lake  is  frequently  termed  a  swamp  during  dry  weather  when 
the  water  is  low.  Many  lakes  are  partly  enclosed  by  swamps,  and 
finally,  as  will  be  seen  later,  the  ultimate  stage  of  a  lake  is  a  swamp 
condition  which  later  usually  changes  to  arable  land.  Lakes  and  swamps 
are  of  great  agricultural  interest  since  lake  beds  and  former  swamps 
now  constitute  large  areas  of  land  and,  furthermore,  the  reclamation 
of  swamps  is  an  important  present  and  future  problem. 

Kinds  of  Lakes 

Lakes  are  bodies  of  water  more  or  less  enclosed  by  land;  they  lie  in 
depressions  or  basins  which  are  usually  somewhat  shallow  although  a 
few  lakes  are  very  deep.  It  is  convenient  to  classify  lakes  according 
to  the  origin  of  their  depressions  and  this  involves  a  brief  review  of 
processes  that  have  been  discussed  in  preceding  pages. 

Glacial  Lakes. — By  far  the  most'  important  process  in  producing 
lakes  is  glaciation.  Indeed  so  characteristic  are  lakes  of  many  glaciated 
regions  that  such  regions  can  often  be  mapped  by  the  abundance  of  small 
irregularly  shaped  lakes.  Small  rock  basin  lakes  occupy  depressions 
that  have  been  excavated  by  glaciers;  they  are  especially  characteristic 
of  mountain  glaciation  and  have  little  agricultural  interest.  Morainic 
lakes  by  the  thousands  occupy  undrained  depressions  in  moraines.  On 
terminal  moraines  they  are  usually  small,  roundish  in  outline  and  often 
without  inlets  or  outlets,  Fig.  221;  many  are  the  well-known  "  kettle 
lakes."  Lakes  on  the  ground  moraine  are  usually  larger.  They  occupy 
shallow  depressions  and  are  typically  "  strung  along  "  a  stream  in  a 
series  of  lakes  and  swamps.  Finally  there  are  many  glacial  lakes  and 
ponds  which  do  not  fall  strictly  under  any  of  the  preceding  classes  but 

238 


KINDS  OF  LAKES 


239 


FIG.  221. — Morainic  lake .  occupying  a  de- 
pression in  a  terminal  moraine,  Montana. 
(Parks,  U.  S.  Geological  Survey.) 


they,  like  all  glacial  lakes,  are  due  to  a  deranged,  immature  drainage. 
The  soils  of  the  former  marginal  glacial  lakes,  which  formed  in  front 
of  the  ice  and  drained  away  as 
the  ice  melted  back,  are  es- 
pecially important.  A  few  of 
these  lakes,  like  Lake  Agassiz, 
were  large,  but  there  were  many 
hundreds  of  smaller  ones.  The 
aggregate  area  of  these  old  lake 
beds  included  millions  of  acres 
and  their  level  surfaces  and 
productive  soils  make  them  very 
important  agricultural  areas,  Fig. 
222. 

River  lakes  are  often  numer- 
ous along  streams  that  meander 

in  a  flood  plain.     There  are  the  crescent-shaped  or  "  ox-bow  lakes," 
Fig.  130,  and  the  irregular  "  sloughs  "  that  occupy  depressions  in  the 

flood  plain.  One  may  find  all 
gradations  in  the  filling  of  river 
lakes,  ranging  from  navigable 
lakes,  swamps  and  filled  swamps 
whose  former  existence  is  often 
indicated  by  their  characteristic 
soils,  Fig.  131.  Delta  lakes  are 
well  shown  in  the  Mississippi 
Delta,  Fig.  145,  the  chief  of 
which  is  Lake  Pontchartrain,  a 
broad  lake  seldom  over  15  feet 

in  depth.     Delta  lakes  because  of  their  shallowness  offer  a  promising 
field  for  reclamation. 

Coastal  Plain  Lakes. — Shallow  lakes  are  likely  to  accumulate  in 
the  depressions  of  a  gently  rolling  surface  such  as  are  found  in  many 
coastal  plains,  which  are  recently  upraised  sea  bottoms  and  have  not 
been  much  eroded.  Many  of  these  lakes  have  been  converted  into 
swamps  locally  called  "  pocosins,"  and  still  others  have  passed  the 
swamp  stage  and  have  become  arable  land. 


FIG.  222. — Principal  areas  of  lake-  soils  in 
the  United  States.  (After  U.  S.  Bureau  of 
Soils.) 


240  LAKES  AND  SWAMPS 


Effects  of  Lakes 

(1)  Lakes  equalize  temperatures,  moderating  both  the  cold  of  the 
winter  and  the  heat  of  the  summer.  The  grape  belt  along  the  southern 
shore  of  Lake  Erie  and  the  peach  belt  along  the  eastern  shore  of  Lake 
Michigan  are  cases  in  point ;  the  cool  winds  from  the  water  in  the  spring 
retard  the  blooming  of  plants  until  danger  from  frost  is  past  and  in  the 
fall  the  warm  winds  delay  the  coming  of  frost.  The  sugar  cane  on  the 
southern  sides  of  some  Louisiana  lakes  is  greener  than  on  the  northern 
side  because  of  the  moderating  effects  of  the  lakes  on  northerly  winds. 
(2)  By  acting  as  natural  reservoirs,  lakes  regulate  the  flow  of  streams. 
A  flood  stream  flowing  into  a  lake  finds  a  larger  channel  in  which 
to  expand  and  so  the  water  is  distributed.  (3)  In  dry  seasons  the 
water  from  lakes  escapes  rather  slowly  and  so  preserves  the  flow  of 
streams.  The  principle  of  artificial  storage  lakes  is  advocated  by  some 
as  a  means  of  flood  control. 

(4)  Lakes  act  as  great  settling  tanks.  Inflowing  streams,  upon 
reaching  the  quiet  waters  of  a  lake,  lose  their  velocity  and  their  load 
of  sediment  is  deposited.  A  classical  example  of  this  function  of  lakes 
is  Lake  Geneva.  The  Rhone  enters  the  lake  a  turbid,  muddy  river 
and  emerges  as  a  stream  of  remarkable  clarity.  Again,  the  streams 
entering  Lake  Erie  carry  loads  of  sediment  and  some  of  them  have  built 
under-water  deltas  of  considerable  size.  On  the  other  hand,  the  Niagara 
River,  which  drains  Lake  Erie,  is  a  clear  river,  so  clear,  indeed,  that  the 
underlying  rock  bottom  can  be  seen  as  the  river  rushes  over  the  American 
Falls. 

Shore  Regions  of  Lakes 

Waves  and  currents  are  commonly  more  often  associated  with 
oceans  than  with  lakes,  for  ocean  waves  attain  much  the  larger  dimen- 
sions. Nevertheless,  waves  on  lakes  of  even  moderate  size  may 
attain  considerable  heights  and  accomplish  notable  work.  Thus  the 
old  lake  (Lake  Bonneville)  that  formerly  occupied  a  portion  of  the 
Great  Basin  cut  cliffs  and  .built  shore  lines  which  remain  to-day  as 
notable  features  long  after  the  old  lake  has  disappeared.  Wave  and 
shore  current  work  of  extinct  lakes  is  of  especial  agricultural  interest 
since  many  former  lake  shores  are  now  occupied  by  arable  soils. 


WAVES  241 


Waves 

As  a  wave  approaches  a  shelving  shore,  the  lower  part  is  retarded 
by  friction  on  the  bottom  while  the  upper  part  continues  at  its  former 
speed  with  the  result  that  the  wave  "  tips  over  "  or  "  breaks,"  forming 
the  breakers  or  surf,  Fig.  244.  The  waters  of  the  surf  run  back  sea- 
ward beneath  the  incoming 
waves  making  an  under-water 
current  called  the  undertow, 
Fig.  223.  It  is  at  once  evi- 
dent that,  when  waves  are 
running  high,  they  are  an  ex-  FIG.  223.— Diagram  to  illustrate  wave  and 
tremely  efficient  agent  of  erosion  current  work.  The  arrows  show  the 
in  the  surf  and  undertow,  for  direction  of  the  undertow, 
the  large  waves  break  with  a 

force  of  tons  per  square  foot,  loosening  and  breaking  shore  materials, 
some  of  which  the  undertow  carries  away. 

The  work  of  these  three  agents,  wave,  surf  and  undertow,  is  thus 
both  destructive  and  constructive,  the  former  activity  usually  being 
the  most  conspicuous.  When  a  wave  breaks  upon  a  shore,  it  loosens 
debris,  some  of  which  is  removed  by  the  undertow,  but  much  is  moved 
back  and  forth,  thus  rounding  the  materials  and  grinding  them  to  smaller 
and  smaller  dimensions,  Fig.  243.  Thus  it  is  that  old  shores  are  almost 
invariably  sandy  or  gravelly.  The  undertow  rapidly  loses  its  velocity 
as  it  moves  toward  deeper  water  and  it  can  carry  only  fine  materials  to 
any  considerable  distances;  the  coarse  materials  are  left  near  shore. 
As  a  result,  there  are  under  some  conditions  the  following  features 
due  to  wave  work:  the  surf  beating  upon  the  shore  cuts  a  cliff;  the 
rapid  undertow  near  the  shore,  armed  with  gravel  and  sand,  cuts  a 
smooth  somewhat  sloping  wave-cut  terrace,  and  farther  out  the  undertow 
constructs  a  wave-built  terrace,  Fig.  224.  Owing  to  various  complications 
these  three  forms  are  often  lacking  and  are  usually  more  or  less  merged 
into  a  sandy,  gravelly,  sloping  beach.  Sandy  and  gravelly  soils  mark 
the  old  shores  of  many  extinct  lakes,  an  example  being  given  in  Fig. 
227,  where  the  gravelly  and  sandy  soils  occur  in  three  belts,  each  belt 
made  at  a  different  height  of  the  former  Lake  Agassiz. 

For  the  sake  of  simplicity  the  waves  have  been  described  as  coming 
straight  on  shore,  but  very  often  they  strike  the  shore  obliquely  as  in 
Fig.  225.  In  such  instances  a  portion  of  the  wave  energy  is  used  to 
produce  a  shore  current  more  or  less  parallel  to  the  coast,  a  current 


242 


LAKES  AND  SWAMPS 


which  carries  the  sand  and  gravel  along  the  coast  and  often  builds  bars 
across  bays  and  inlets.  Sandy  Hook,  which  nearly  encloses  New  York 
Harbor,  is  built  by  southerly  shore  currents;  the  deposits  here  brought 
by  these  currents  require  constant  dredging  to  keep  the  harbor  channel 
open. 


\ 


FIG.  224. — Wave-cut  terrace  of  an  extinct  glacial  lake  at  the  right;   wave-cut  cliff 
at  the  left.     (Alden,  U.  S.  Geological  Survey.) 

Barrier  Beaches  or  Off  shore  Bars. — It  has  been  noted  that  the 
undertow  carries  out  debris  which  it  often  deposits.  Furthermore,  on 
shallow  coasts  the  waves  drag  debris  shoreward  and  often  build  up  a 
low  ridge.  Because  of  these  two  agencies  acting  either  singly  or  together, 

a  low  under- water  ridge  is  often 
built  up  to  the  height  of  the  storm 
waves.  The  winds  catching  up  the 
sand  build  it  into  dunes  and  spread 
the  sand  so  that  a  long,  low  ridge 
is  formed,  often  termed  a  "  sand 
reef  "  or  "  barrier  beach."  Such 
are  common  along  the  south  At- 
lantic and  Gulf  coasts.  The  coast 

of  Texas  is  practically  fringed  by  such  bars  with  only  a  few  breaks 
or  "  inlets."     Galveston  is  built  on  one  of  these  bars,  Fig.  245. 

Shore  Lines  at  Different  Water  Levels. — It  has  been  assumed 
that  the  water  surface  holds  about  the  same  level  while  the  waves  are 
working  and  this  is  practically  true  for  the  sea  level  at  least,  for  a  long 
period,  but,  in  the  extinct  lakes  with  which  we  are  especially  interested 


SHORE  CURRENTS 


FIG.  225. — Diagram  to  illustrate  the  de- 
velopment of  shore  currents. 


WAVES  243 

from  an  agricultural  point  of  view,  the  lake  surfaces  suffered  many 
variations  of  level.  For  example,  nearly  a  dozen  shore  lines  have  been 
identified  around  the  extinct  Lake  Agassiz,  each  recording  a  separate 
level  of  the  lake.  As  these  lakes  were  filling  or  draining  the  waters 
cut  shores  as  they  rose  or  fell,  but  no  marked  shore  lines  were  made 
unless  the  water  surface  was  maintained  at  practically  the  same  level 
for  a  long  time.  The  best-preserved  shore*  features  were  naturally 
made  when  the  lake  waters  were  falling,  since  the  waves  of  a  rising  lake 
would  modify  or  destroy  the  previous  bars,  cliffs  and  other  features. 
The  soils  associated  with  lake  beaches  may  be  roughly  divided  into 
two  classes,  first  those  formed  where  the  lake  surface  was.  at  fairly 
constant  level  so  that  well-defined  shore  features  were  made;  second 
those  formed  where  the  lake  surface  was  shifting  and  the  wave  action 
was  in  consequence  relatively  ineffective  in  producing  well-marked 
shore  features.  The  typical  arrangement  of  soils  in  the  first  case  will 
be  readily  understood  from  the  general  principles  of  wave  work.  The 
breaking  waves  detach  and  loosen  debris  and  roll  it  back  and  forth  while 
the  finer  materials  are  carried  out  by  the  undertow.  Then  at  places 
where  conditions  are  favorable,  the  incoming  waves  build  gravelly 
ridges  which  now  stand  as  prominent  features  above  the  lake  plain, 


FIG.  226. — Beach  ridge  of  an  extinct  glacial  lake,  Mich.     (Leverett,  U.  S.  Geological 

Survey.) 

Fig.  226;  such  ridges  are  very  prominent  in  northern  Ohio  where  they 
are  often  utilized  as  main  roads  across  the  lake  plains.  Typically,  the 
soils  in  such  shore  belts  as  we  are  considering'  show  a  roughly  belted 
arrangement.  (1)  The  belt  of  surf  and  swift  waters  are  marked  by 
coarse-textured  soils  such  as  gravels  and  gravelly  loams.  (2)  On  the 
other  hand,  the  belt  deposited  by  the  undertow  would  show  progressively 


244 


LAKES  AND  SWAMPS 


finer  materials  as  the  distance  from  the  old  shore  increases  and  the  soils 
would  be  mainly  loams  and  sandy  loams  while  (3)  in  the  deeper,  quiet, 
waters,  clays  and  clay  loams  would  predominate. 

Such  an  ideal  arrangement  is  seldom  to  be  found,  but  a  large  area  of  soils  shown  in 
Fig.  227  exhibits  this  arrangement.     The  soils  due  to  the  active  waters  of  the  surf 


SMwrafijLa^^iwBMi.— 


GRAVEL  ooooo    GRAVELLY  LOAM  oXoX   LOAMS     XXX    CLAY,  CLAY  LOAM  &  SILT  LOAM  SANDY  LOAM  ';*•'.•  '.v'.vV 


FIG.  227.  —  Shore  and  deep-water  soils  of  the  extinct  glacial  Lake  Agassiz.     (After 

U.  S.  Bureau  of  Soils.) 


T  C 


and  shore  waters  are  gravelly;  this  gravelly  belt  changes  to  the  loams  deposited  by 

the  undertow  and  lastly,  clays,  silts  and  silt  loams  were  deposited  in  the  deeper, 

more  quiet  waters  of  the  lake. 

A  soil  arrangement  illustrating  the  second  class  in  which  the  shore  was  shifting 

is  illustrated  in  Fig.  228.     It  should,  however,  be  remembered  this  area  in  places 

also  includes  soils  made  by  water  that 
was  more  or  less  at  a  constant  level;  in 
fact,  the  two  classes  of  soils  are  very 
seldom  entirely  distinct  for  any  consid- 
erable area.  A  broad  belt  of  sandy  soils 
(LS)  was  made  by  advancing  and  reced- 
ing wave  action  with  here  and  there 
gravelly  and  sandy  ridges  built  by  the 
waves  when  the  water  level  was  station- 
ary for  some  time.  This  belt  is  coarse- 
textured  because  the  finer  materials  were 
carried  to  deeper  waters.  Where  the 
wave  zone  moved  over  areas  of  boulder 
clay,  much  of  the  finer  materials  were 
removed  and  the  stones  left,  thus  re- 
sulting in  stony  clays  (LCT).  On  the 
other  hand,  where  the  materials  were 


T  C 


-     10  Miles     ,— \ 


T   C 


FIG.  228. — Lacustrine  soils  deposited  in 
the  former  Lake  Agassiz  in  north- 
western Minnesota.  The  dark  lines 
are  beach  ridges.  (After  Leverett, 
Minnesota  Geological  Survey.)  . 


sandy  (sandy  till)  the  currents  removed 
most  of  the  clay,  leaving  sandy  soils 
(LST).  The  larger  area  of  lacustrine 
soils  is  found  as  usual  where  the  waters 
were  deep  and  quiet  and  the  finer  ma- 
terials settled  furnishing  clays  and  clay 
loams  (LC).  TC  and  TM  in  the  map  represent  upland  soils,  the  former  some- 
what clayey  and  the  latter  somewhat  sandy.  It  will  be  noted  that  the  ridges  are 
often  discontinuous  and  that  they  branch  frequently  and  not  infrequently  they 
change  directions.  Such  ridges  are  usually  most  distinct  where  winds  are  strongest 


WAVES 


245 


and  most  persistent  for,   under  such  conditions,  wave  action  is  most  effective, 
other  things  being  equal.     Shore  soils  also  show  much  variation  according  to  the 
materials  of  the  beach,  wave  strength,  wind  persistence  and  direction  shore  current® 
and,  of  course,  the   persistence  of 
the  lake  level  which  necessarily  de- 
termines how  long  the  waves  can 
work. 

Deltas  are  often  conspicu- 
ous shore  features  of  lakes 
especially  since  there  are 
seldom  waves  and  currents 
strong  enough  to  interfere 

greatly   with    delta    building;      FIG.  229. — One  of  the  series  of  level-topped 
as  a  result  even  small  streams        deltas  built  one  above  the  other  at  differ- 

have  built  considerable  deltas.         !f  flakfe  £j*j  *'  Y'    ™°  road  1™.  a* 
_.  .  the  foot  of  the  delta.     (O.  D.  von  Engeln.) 

These  deltas  of  extinct  lakes 

often  rise  along  valley  sides  in  a  series  of  level-topped  deltas  much 
like  huge  steps,  Fig.  229,  each  step  having  been  built  at  different  levels 
of  the  water.  Deltas  and  their  soils  have  been  considered  in  fore- 
going paragraphs  and  do  not  call 
for  more  extended  notice  here, 
except  to  call  attention  to  the 
soils  of  a  delta  which  was  built 
in  the  extinct  Lake  Agassiz,  Fig. 
230,  a  delta  which  will  serve  as 
an  example  of  many.  The 
Sheyenne  River,  now  a  tributary 
of  the  Red  River,  was  a  much 
larger  stream  when  it  flowed  into 
Lake  Agassiz  and  built  a  delta 
that  now  covers  some  800  square 
miles.  The  delta  building  oc- 
curred at  one  of  the  earlier 
levels  of  the  lake  and,  with  the 

Sheyenne  River  built  into  the  extinct  falling  °f  the  lake  waters>  the 
Lake  Agassiz.  (After  Upham,  U.  S.  Geo-  river  cut  a  channel  through  its 
logical  Survey  and  U.  S.  Bureau  of  Soils.)  delta.  The  soil  particles  show 

a  progressive  decrease  in  size 

from  the  head  of  the  delta  towards  the  delta  margin,  a  decrease  due  to 

the  diminishing  velocity  of  the  delta  building  stream. 


DELTA  SOILS 

1  SAND   .'.•.'.'.' 

2  FINE  SAND   .V.V.' 

3  FINE  SANDY  LOAM  v 

10  Miles 


LAKE  SOILS 

4  LOAM  ^v^xs.NN 

5  CLAY  LOAM^-3- 


FIG.  230.— Soils  of  the  old  delta  which  the 


246 


LAKES  AND  SWAMPS 


SBORE  SOJLS 

LIMESTONE  MATERIALS       rr      ---,t1M.. 
SANDSTONE  AND  SHALE  MATERIALS... 


Lake  Deposits  and  Lake  Basins 

There  is  naturally  a  close  relation  between  lake  deposits  and  the 
materials  forming  their  basins.  Sandy  basins  will  ordinarily  yield 
sandy  lake  sediments  and  a  basin  of  clay  or  calcareous  materials  will 

yield  somewhat  characteristic 
lake  sediments.  This  is  well- 
illustrated  on  a  large  scale  in 
the  soils  now  occupying  the  bed 
of  the  former  glacial  Lake  Mau- 
mee  in  northwestern  Ohio,  Fig. 
231.  Most  of  the  tributary 
upland  is  covered  with  calcare- 
ous soils  derived  from  limestones 

and  the  lake  soils,  especially  the 
FIG.  231.— Shore  soils  around  a  portion  of      old  shore  goils  ghow  fragments 

the  extinct  glacial  Lake  Maumee.     Lime-        £   ,.  ,    ,,  ,      ., 

•  i     •  ,  i     i  -i       j     of  limestone    and  the  subsoils 

stone  materials  yield  calcareous  soils  and 

sandstone  and  shale  materials,  sandy  soils.      are    usually    alkaline.      In    the 
(After  Ohio  State  Soil  Survey.)  northeastern  part  of  the  basin 

the  upland    soils   are    largely 

from  shales  and  sandstones  and  in   consequence  the  shore  soils  and 
the  corresponding  lake  sediments  are  much  more  sandy. 

It  is  important  to  note  that  the  shore  soils  of  lakes  show  a  much 
closer  relation  to  the  materials  of  the  basin  than  the  soils  derived  from 
deep-water  sediments.  The  finer  silts  and  clays,  which  are  deposited  in 
deep  waters,  are  common  to  many  soils  derived  from  greatly  differing 
rocks  and  the  origin  of  these  sediments  either  in  rivers  or  lakes  is  usually 
difficult  to  trace.  Probably  the  most  characteristic  of  these  finer 
sediments  are  those  of  calcareous  origin  and  soils  from  such  sediments 
often  show  a  high  lime  content.  Shore  materials  from  granite,  shale, 
sandstone  and  other  rocks  are  usually  easily  identified  by  the  pebbles. 
Weak  rocks,  like  shales  and  some  sandstones,  ordinarily  furnish  finer 
shore  materials  than  strong  rocks  such  as  granites.  Furthermore,  shore 
materials  from  such  igneous  rocks  as  granite  usually  furnish  soils  with 
higher  contents  of  lime  and  potash  than  those  from  shales  and  sand- 
stones and  the  shore  soils  built  of  limestone  materials  are  usually  more 
or  less  calcareous.  The  materials  in  the  basins  of  small  lakes  are  far 
more  likely  notably  to  influence  the  lake  sediments  than  in  large  lakes, 
since  large  lakes  usually  receive  a  greater  variety  of  sediments  and 
there  is  more  mixing  of  sediments.  Delta  and  river  lakes  usually  yield 


TOPOGRAPHY  OF  LAKE  BOTTOMS 


247 


fine-grained  soils  because  the  streams,  as  a  rule,  are  carrying  finer 
materials  in  their  lower  courses  where  these  lakes  are  found. 

Extinction  of  Lakes 

It  has  been  noted  that  lakes  are  relatively  short-lived  geological 
features.  They  become  extinct  in  two  ways,  by  draining  and  by  filling. 
As  the  outlet  of  a  lake  is  lowered  by  cutting  the  outlet  channel,  the 
water  is  drained  from  the  basin  and  the  surface  is  lowered.  On  the 
other  hand,  lakes  are  filled  (1)  by  inwash  from  the  sides,  (2)  by  deposits 
by  inlets  and  (3)  many  lakes  are  more  or  less  filled  by  accumulations 
of  vegetable  matter  in  them.  Obviously  these  three  agencies  of  extinc- 
tion may  and  often  do  operate  simultaneously.  Since  there  is  no  sharp 
distinction  between  lakes  and  swamps  and,  moreover,  since  lakes  tend 
to  become  swamps  in  later  stages  of  their  filling,  the  discussion  of  this 
topic  will  be  taken  up  in  detail  under  the  topic  of  swamps.  It  is  suf- 
ficient to  note  here  that  there  are  thousands  of  small  lakes  which  have 
been  filled  or  drained  and  have  become  arable  land  and,  as  population 
becomes  more  dense,  many  lakes  and  swamps  will  be  reclaimed  and  their 
soils  utilized. 

Topography  of  Lake  Bottoms 

The  most  notable  topographic  feature  of  exposed  lake  bottoms  is 
their  level  surface,  forming  lake  plains  which  are  due  to  the  slow,  even 

r '  • 


FIG.  232 — The  plain  of  Lake  Agassiz,  North  Dakota.  The  distant  trees  mark  the 
course  of  the  Sheyenne  River.  (W.  C.  Palmer,  North  Dakota  Agricultural 
College.) 

accumulation  of  fine  materials  which  settle  to  the  bottoms  of  the  lakes. 
In  time,  if  the  lake  exists  long  enough,  any  original  inequalities  of  the 
bottom  become  buried,  but  not  infrequently  some  higher  hills  are  not 
covered  and  now  rise  through  the  lake  sediments  above  the  surrounding 
plain.  The  surface  of  Lake  Agassiz  plain,  Figs.  212  and  232,  is  so  level 


248  LAKES  AND  SWAMPS 

that,  owing  to  the  earth's  curvature,  the  tops  of  elevators  and  other 
buildings  are  first  seen  as  they  are  approached,  an  effect  often  observed 
at  sea.  Beaches,  of  course,  do  not  have  the  level  topography  of  the 
plain  formed  in  deeper  waters. 

Lacustrine  or  Lake-made  Soils 

are  usually  productive.  The  most  extensive  connecting  areas  of  these 
soils  are  the  exposed  bottoms  of  the  extinct  marginal  glacial  lakes, 
because  these  lakes  have  entirely  disappeared  while  most  other  glacial 
lakes  have  not  yet  been  filled  or  drained.  Filled  river  lakes  are  not 
uncommon  and  some  soils  of  partly  filled  coastal  plain  lakes  have  been 
utilized. 

The  level  topography  of  lake  plains  produces  three  important 
agricultural  factors:  (1)  The  drainage  problem  both  surface  and  sub- 
surface is  important.  (2)  The  level  surface  and  practical  absence  of 
stones  allows  the  fullest  use  of  agricultural  machinery.  (3)  Another 
somewhat  characteristic  feature  of  many  lacustrine  soils,  especially  in 
humid  climates,  is  their  high  humus  content  due  to  their  slow  drainage. 

Lacustrine  soils  as  a  whole  are  fine-textures,  mostly  silts  and  clays, 
because  the  largest  areas  were  deposited  in  quiet  waters.  The  shore 
regions  are  more  sandy.  Beach  soils  vary  from  gravels  to  loams  accord- 
ing to  the  materials  with  which  the  waves  worked,  to  the  strength  of 
the  waves  and  shore  currents  and  to  the  length  of  time  during  which 
these  agents  operated.  There  is  a  tendency  for  lacustrine  soils  to  have 
a  roughly  concentric  arrangement,  ranging  from  the  fine-textured  soils 
of  the  deep  waters  to  the  coarser-textured  soils  of  the  beach  and  shallow- 
water  areas,  but  the  arrangement  is  often  very  irregular.  Many  river 
and  delta  lakes  are  exceptions  to  the  above  statements  for  they  are  in 
part  filled  by  overflows  from  the  rivers  and  such  river  sediments  are 
usually  fine  so  that  these  soils  are  typically  clays,  clay  loams  and  silt 
loams.  In  general,  the  soils  of  small  extinct  lakes  are  less  variable 
than  those  of  larger  lakes,  since  their  deposits  are  more  closely  associated 
with  the  lake  basins  and,  furthermore,  there  is  usually  less  distinction 
between  shore  and  deeper  water  deposits,  because  wave  action  on  small 
lakes  is  usually  weak. 

Saline  Lakes 

These  are  found  in  regions  of  deficient  rainfall  when  lakes  have  no 
outlets.  The  salt  (NaCl)  and  other  soluble  materials,  which  are  carried 


SALINE  LAKES 


249 


into  the  lakes,  are  left  behind  when  the  waters  are  evaporated  and  as 
this  process  goes  on,  the  salts  keep  accumulating  until  the  waters 
become  saturated  for  some  substances  and  deposition  begins.  This 
process  has  agricultural  in- 
terest for,  as  will  be  seen 
later,  some  potash  fertilizers 
are  believed  to  have  accumu- 
lated under  similar  conditions. 
The  Great  Salt  Lake,  Fig. 
233,  is  of  interest  in  this  con- 
nection. Not  long  ago,  geo- 
logically speaking,  there  were 
several  fresh  water  lakes  in 
the  Great  Basin  of  which  Lake 
Bonneville  was  the  predecessor 


FIG.  233. — Areas  formerly  covered  by  the 
extinct  Lakes  Bonneville  and  Lahontan  in 
the  Great  Basin.  Great  Salt  Lake  is  a 
remnant  of  Lake  Bonneville. 


of    the   present     Great     Salt 
Lake.      The    waters    of    this 
fresh-water  lake  flowed  north- 
ward into  the   Snake   River,   cutting   a   deep  valley,  and  the  waves 
formed  hundreds  of  miles  of  shore  forms  which  are  now  almost  as 


FIG.  234. — View  across  an  arm  of  the  extinct  Lake  Bonneville,  Utah.     (U.  S.  Bureau 

of  Soils.) 

perfect  as  when  they  were  built,  their  preservation  being  due  both  to 
their  recency  and  to  the  scanty  rainfall  of  the  region.     The  study  of 


250  LAKES  AND  SWAMPS 

these  shore  lines,  now  so  well  exposed  and  easily  examined,  has  thrown 
much  light  on  our  knowledge  of  shore  forms.  As  the  rainfall  of  the 
region  diminished,  Lake  Bonneville  began  to  shrink  and  when  the 
waters  fell  below  the  surface  of  the  outlet,  the  waters  became  increas- 
ingly salty  until  the  present  Great  Salt  Lake  remains  a  remnant  of  the 
much  larger  fresh  Lake  Bonneville. 


SWAMPS,   CUMULOSE  SOILS 

Swamps  occur  when  land  is  covered  with  shallow  water  for  all  or 
a  considerable  part  of  the  year  or  when  the  soil  is  kept  wet  much  of 
the  time  so  that  vegetation  in  falling  does  not  completely  decay.  Swamps 
are  interesting  from  an  agricultural  point  of  view  (1)  because  many  soils 
are  derived  from  swamp  deposits;  such  soils  are  called  cumulose  soils. 

(2)  Many  .prairie  soils  have  been  subjected  to  semi-swamp  conditions 
so  that  they  are  well  supplied  with  humus.     (3)  Again,  the  problem  of 
swamp  and  marsh  reclamation  is  extremely  important  in  many  regions. 
(4)  Finally,  swamps  are  of  further  interest  in  that  beds  of  peat  and  coal 
were  formed  under  swamp  and  marsh  conditions  and  a  study  of  present 
swamps  helps  in  understanding  these  past  conditions. 

Factors 

Some  conditions  affecting  the  formation  of  swamps  are  as  follows: 
(1)  Topography;  flat  land  from  which  the  run-off  is  slow  and  areas 
depressed  below  surrounding  areas  so  that  drainage  is  towafd  the  swampy 
areas  are  both  favorable  to  swamp  conditions.  (2)  Rainfall  and  evap- 
oration are  obviously  important  factors;  in  an  arid  region,  swamps  are 
infrequent  although  some  occur  and  are  supplied  with  water  apart  from 
local  rainfall.  Rapid  evaporation  evidently  militates  against  swamps. 

(3)  The  amount  and  nature  of  vegetation  is  important  because  vegeta- 
tion tends  to  retain  moisture  and  so  promote  swampy  conditions. 
Indeed,  under  some  conditions  an  abundant  growth  of  vegetation  may 
in  itself  bring  on  swampy  conditions.     (4)  The  porosity  of  the  subsoil 
or  underlying  formations  obviously  affects  the  downward  movement 
of  waters;  most  large  swampy  areas  are  underlain  by  relatively  imper- 
vious clays  or  silts  which  retard  the  downward  escape  of  water. 


CLASSES  OF  SWAMPS  251 

Classes  of  Swamps 

The  classes  of  swamps  correspond  closely  to  those  of  lakes  because 
lakes  and  swamps  grade  into  each  Other.  A  common  usage  is  to  restrict 
the  term  swamp  to  those  of  fresh-water  origin  while  similar  features  of 
salt  water  origin  are  often  termed  marshes.  Although  there  is  often  a 
close  connection  between  lakes  and  swamps,  it  does  not  necessarily 
follow  that  all  swamps  were  preceded  by  lakes  or  ponds,  although  many 
swamps  have  had  these  predecessors.  Swampy  or  semi-swampy  con- 
ditions may  result  from  poor  drainage,  an  increase  in  rainfall  or  by  an 
increased  growth  of  some  types  of  vegetation  which  retain  moisture 
and  lead  to  swampy  conditions,  an  illustration  being  the  so-called 
climbing  bog,  which  by  rank  growth  of  vegetation  may  spread  beyond 
the  original  borders  and  so  extend  a  swamp  area. 

Glacial  swamps,  like  the  glacial  lakes,  are  most  numerous.  The 
glaciers  left  many  shallow,  undrained  depressions  and  a  drainage  so 
disorganized  that  these  depressions  in  most  instances  are  as  yet  undrained 

and    remain     as     lakes     and  

swamps.    Most  morainic  lakes 


are  shallow,  many  have  been 
completely  filled  since  the 
Glacial  Period,  while  thou- 
sands of  others  are  yet  in 

various    swamp    stages,    Fig.      ,,     00_     ~.    .  ,  ,  .  ,         ,       ,   „ 

FIG.  235. — Glacial  lakes  and  ponds  wholly 

235.        Large       swamps      are         Or  partly  filled,  North  Dakota;  dotted  areas 

found  on    many  ground    mo-         indicate  lacustrine  soils.      (U.  S.  Bureau 

raines  where  water  stands  in         of  Soils.) 

broad,     shallow     depressions, 

some  of  which  are  completely  filled  and  are  covered  with  deep,  black 

soil,  Fig.  236. 

Alluvial  swamps  are  of  great  present  and  potential  importance  espe- 
cially along  the  larger  rivers  where  they  have  formed  on  the  back  lands 
some  distance  away  from  the  streams.  The  alluvial  swamps  along  the 
lower  Mississippi  extend  for  long  distances  with  but  little  interruption. 
Alluvial  swamps  may  be  roughly  divided  into  two  classes.  (1)  The  ox- 
bow lakes  along  rivers  have  been  noted,  page  158;  these  in  time  reach 
the  swamp  condition  and  later  become  filled.  (2)  The  larger  and  more 
important  swamps  are  found  in  the  low  back  lands  where  drainage  is 
sluggish.  It  is  these  river  swamps  which  offer  fairly  easy  reclamation 
and,  when  reclaimed,  they  furnish  very  productive  soils.  These 


252 


LAKES  AND  SWAMPS 


swamps  now  are  for  the  most  part  wooded  and  the  vegetation  helps  to 
fill  them  by  catching  the  sediments  brought  by  the  waters. 


FIG.  236. — Onions  on  muck  soil.    A  filled  glacial  swamp,  N.  Y.     (U.  S.  Bureau  of 

Soils.) 

Coastal  Plain  Swamps. — Swamps  and  swampy  areas  are  common  on 
the  level  areas  of  the  Coastal  Plain,  especially  in  the  "  flatwoods"  and 

prairies  of  this  region  in  Virginia 
and  the  Carolinas.  These  areas 
have  the  "  appearance  of  a  dead- 
level  plain  varied  occasionally 
by  slight  hollows  and  ridges  and 
by  shallow  valleys"  (Bennett). 
In  some  places  the  slopes  are  so 
gentle  that  the  ground  water  is 
at  or  near  the  surface  and  swampy 
conditions  result;  in  other  places 
swamps,  often  locally  called 
"  pocosins  "  form  in  slightly  de- 
pressed areas.  The  Great  Dismal 
Swamp  of  Virginia  and  South 
Carolina,  Fig.  237,  is  an  example 


FIG.  237.— Map  of  Dismal  Swamp,  Va.        of  a  coastal  plain  swamp.     The 

swamp  has  formed   in   a    slight 

depression  and  near  the  center  is  Lake  Drummond,  which  is  apparently 
a  portion  of  a  lake  once  larger,  but  which  has  been  filled  by  encroaching 


FILLING  OF  LAKES  AND  SWAMPS 


253 


FIG.  238.— Vegetation  filling  a  lake.  The 
old  shore  line  was  formerly  at  the  hill 
on  the  left.  (Fenneman,  Wis.  Geological 
Survey.) 


vegetation.     In  many  places  on  the  flat  Coastal  Plain,  only  the  occur- 
rence of  peaty  or  humus  areas  shows  the  former  existence  of  a  swamp. 

Filling  of  Lakes  and  Swamps 

There  is  little  essential  differ- 
ence in  the  filling  of  lakes  and 

swamps,     Both  are  subject  to 

some  filling  by  in  wash,  although 

this    process    is    usually  more 

prominent    in    lakes    than    in 

swamps.     On  the  other  hand, 

the  work  of  vegetation  in  filling 

is  on  the  whole  more  important 

in  swamps  than  in  lakes,  mainly 

because   swamps    are    generally    shallow  so   that   dense   vegetation, 

rooted  in  the  bottom,  thrives  better  in  swamps. 

The  filling  of  a  lake  and  swamp  is  illustrated  in  Fig.  239.     In  the 

zone  of  shallow  water  about  the 
shores,  aquatic  rooted  plants 
such  as  reeds,  canes,  grasses 
and  some  shrubs  will  begin^to 
grow.  The  leaves,  twigs  and 
stems  of  these  plants  fall  into 
the  water,  partially  decay,  sink 
to  the  bottom  and  shallow  the 
water  until  rooted  trees  can  get 
a  foothold.  The  process  is  re- 
peated until  the  soil  becomes 
dry  enough  for  dry  land  trees 
and  the  marginal  parts  of  the 
swamp  or  lake  become  essen- 


K  -  • 

y/^ 


FIG.  239.— Diagram  illustrating  the  filling 
of  lakes  by  vegetation.  (1)  Open  water 
and  aquatic  plants;  (2)  marginal  and 
shore  plants;  (3)  swamp  meadow;  (4) 
swamp  shrubs;  (5)  swamp  forest;  (6) 
upland  forest.  (After  Dachnowski,  Ohio 
Geological  Survey.) 


tially  uplands.  Roots  and  stems 
also  arrest  the  sediment  washed 
in  and  this  is  added  to  the  soil 
that  is  forming. 

Meanwhile,  floating   plants 
such  as  mosses  which  float  on  the 

surface  and  thread-like  plants  (algae)  which  grow  on  the  bottom  are  also 
filling  the  deeper  parts  of  the  lake  or  swamp.  Floating  mats  of  such  plants 
and  of  shore  plants  are  common  features;  these  mats  often  attain  con- 


254 


LAKES  AND  SWAMPS 


siderable  area  and  thickness  and  in  time  become  waterlogged  and  sink 
to  the  bottom,  thereby  building  up  the  bed  of  the  swamp  or  lake.  Fig. 
239  shows  three  stages  in  lake  filling  by  vegetation;  on  the  right  are 
steep  sides  and  on  the  left  are  gently  sloping  sides.  In  the  first  figure 
(A),  the  vegetation  has  filled  the  shallow  water  and  built  up  a  peaty 
matting  out  to  the  deeper  water.  The  zones  of  upland,  swamp  and 
of  water  vegetation  encroach  on  the  lake  until  in  the  last  stage  (C)  the 
lake  or  swamp  has  been  converted  into  land.  Thus,  many  lakes  and 
swamps  in  process  of  filling  show  several  zones.  The  shore  waters 
become  shallow  and  finally  form  land  which  supports  land  vegetation. 
In  front  of  this  is  a  bog  with  trees,  shrubs,  grasses  and  reeds.  Fronting 
this  is  a  low,  swampy  shore  and  finally  more  or  less  open  water  with 
floating  mats  of  vegetation  and  this  in  time  becomes  partly  filled  with 
vegetable  matter  and  becomes  a  "  quaking  bog."  Finally  the  entire 
area  may  become  firm  land  and  the  zonal  arrangement  of  vegetation 
disappears.  All  stages  are  found  from  lakes  and  swamps  which  have 
but  narrow  zones  of  encroaching  vegetation  to  those  which  are  entirely 
changed  to  arable  land. 

In  the  warmer  climate  of  the  South,  mosses  are  not  important  agents 
in  lake  and  swamp  filling.     Here  the  grasses,  reeds,  "  cane  "  and  dwarf 

palmettos,  all  rooted  plants,  form  a  dense 
growth  in  places,  according  to  Shaler,  of  50 
to  75  plants  to  a  square  foot.  The  work  of 
the  mangrove  trees  is  especially  effective  in 
some  southern  swamps,  particularly  in  the 
Everglades  of  Florida.  The  branches  of  this 
tree  bend  down  and  take  root  in  the  swamp 
bottom  so  that  there  is  a  zone  of  vegetation 
advancing  on  the  water.  The  dense  growth 
catches  sediment,  the  falling  leaves  and 
twigs  are  added  to  the  sediment  and  the 
swamp  fills  up  from  the  shores.  The  Ever- 
glades of  Florida  is  a  good  example  of  these 
swamps,  Fig.  240.  They  "  owe  their  exist- 
ence primarily  to  an  abundant  rainfall 
and  to  the  slight  elevation  of  southern 
Florida.  Even  were  there  no  basin-like 
structure  whatever,  and  were  the  bed 

rock  absolutely  flat,  the  present  rainfall,  the  sluggish  drainage 
and  the  luxuriant  growth  of  vegetation  would  result  in  a  swamp  " 


FIG.  240.— The  Everglades 
of  Florida.  Lake  Oke- 
chobee  lies  within  the 
Everglades.  (After  Sel- 
lards.) 


LAKE  AND  SWAMP  DEPOSITS  255 

(Sellards).    A  section  of  a  portion  of  the  Everglades  is  shown  in 
Fig.  241. 

Lake  and  Swamp  Deposits 

Marl  and  Bog  Lime. — Bog  lime  is  a  deposit  of  calcareous  materials 
in  lakes,  bogs  or  swamps;  it  is  often  incorrectly  termed  marl.  Strictly 
speaking,  marl  is  a  calcareous  clay  which  varies  considerably  in  compo- 
sition. Marl  beds  are  not  infrequently  found  in  lakes  and  swamps, 
the  marl  being  in  some  cases  highly  calcareous.  To  some  extent  marl 
and  bog  lime  are  caused 
by  the  accumulation  of 
shells,  especially  minute 
shells,  as  they  sink  to  the  LIMESTONE  ^a 
bottom.  However,  it  is  FlG>  241.— Section  of  a  part  of  the  Everglades 
believed  that  most  of  these  which  here  occupies  a  shallow  limestone  basin, 
materials  are  secreted  from  (After  Sellards.) 
the  water  by  plants,  par- 
ticularly by  certain  thread-like  plants  (algae)  which  deposit  lime  in 
their  tissues  and  on  their  surfaces.  Both  materials  often  have  a 
composition  suitable  for  cement  and  are  used  for  this  to  some  extent 
for  that  purpose.  They  also  yield  lime  and  are  commonly  used  locally 
for  fertilizers,  especially  when  they  contain  considerable  lime  phos- 
phate, as  is  often  the  case. 

Peat  is  formed  from  the  accumulation  of  partially  decayed  plants 
and  occurs  in  all  stages  from  plant  stems,  roots  and  twigs  but  little 
changed  to  a  much  changed  black,  waxy  substance.  It  is  found  in 
many  swamps  and  lakes,  especially  in  cool  climates.  It  is  due  to 
incomplete  decay  as  vegetation  falls  into  water  and  then  by  the  aid 
of  bacteria  is  changed  into  peaty  substance.  Peat  is  used  agriculturally 
as  an  absorbent  and  has  some  local  use  as  a  fertilizer,  especially  for  sandy 
soils.  In  Europe  peat  is  used  to  a  considerable  extent  as  fuel.  Peat 
is  of  further  interest  in  that  it  represents  one  of  the  early  stages  in  the 
formation  of  coal.  Swamps  are  now  an  important  source  of  timber. 
Owing  to  the  difficulty  of  removal,  swamp  timber  has  until  recently 
been  practically  uncut,  but  this  source  is  now  perhaps  our  greatest 
timber  reserve. 

A  common  manner  of  swamp  and  lake  filling,  together  with  the  accu- 
mulation of  peat  and  marl,  is  shown  in  Fig.  242.  In  the  first  stage  (A) 
there  is  a  growth  of  plants  which  slowly  fill  the  marginal  waters.  In 


256 


LAKES  AND  SWAMPS 


the  deeper  waters  the  small  thread-like  plants  secrete  more  or  less  lime 
which  accumulates  on  the  bottom  as  marl  as  the  plants  die.  Mean- 
while, the  shore  vegetation  is  causing  a  deposit  of  peat  which  accumu- 
lates above  the  marl  so  that  there  is  an  advancing  zone  of  marl  followed 
by  an  overlying  following  zone  of  peat  (B).  A  later  stage  is  shown  in 

(C)  which  is  a  common  oc- 
currence. It  is  evident  that, 
other  things  being  equal, 
marl  will  accumulate  more 
rapidly  where  the  surround- 
ing materials  are  calcareous, 
a  condition  often  found  in 
glacial  materials  where  cal- 
careous rocks  have  been 
ground  up  and  are  therefore 
readily  leached  by  the  ground 
water. 


Cumulose  Soils 


FIG.  242. — Diagram  to  illustrate  the  accumu- 
lation of  peat  and  marl  in  a  filling  lake  or 
swamp.  A  represents  an  early  stage  with 
the  following  plant  zones:  (1)  conifers;  (2) 
bog  shrub;  (3)  bog  hedge;  (4)  aquatic  plants. 
(After  Transeau.) 


These  soils,  like  lacustrine 
soils,  are  characterized  by 
their  high  humus  and  nitro- 
gen content;  they  vary  from 
black,  waxy  soils  to  peaty 

soils  and  to  loams  and  clay  loams  with  high  humus  content.  The  high 
humus  content  is  due,  of  course,  to  the  large  proportion  of  vegetation 
filling  in  swamps  and,  since  this  process  is  more  prominent  in  swamps 
than  in  lakes,  the  cumulose  soils  usually  show  higher  nitrogen  than 
lacustrine  soils. 

These  principles  are  well  shown  in  the  following  table,  which  shows 
high  nitrogen  both  in  soil  and  subsoil  of  recent  cumulose  soils.  It  is 
interesting  also  to  note  that  the  nitrogen  content  is  higher  on  the  flat 
prairies  than  on  the  undulating  prairies,  a  difference  due  mainly  to  the 
slower  drainage  and  consequently  greater  accumulation  of  vegetable 
matter  on  the  flat  prairies. 

The  mineral  content  of  cumulose  soils  varies  considerably,  but  in 
general  the  potash  is  low  as  is  shown  by  the  table  above.  The  phos- 
phoric acid  and  lime  is  often  rather  high  owing  in  large  measure  to  the 
accumulations  of  lime,  secreting  plants  and  animals.  The  mineral  con- 


CUMULOSE  SOILS 


257 


tent,  of  course,  will  vary  with  the  character  of  the  materials  that  are 
washed  or  blown  into  the  swamps.  Furthermore,  where  the  contrib- 
uting vegetation  is  rooted  there  is  naturally  more  mineral  matter 
from  this  source  than  from  floating  plants  since  rooted  plants  are  in 
direct  connection  with  the  under-water  soils. 

TABLE  SHOWING  THE  AVERAGE  NUMBER  OF  POUNDS  PER  ACRE  IN  SOME 
ILLINOIS    SOILS   (0-7  INCHES)    AND  SUBSOILS   (20-40  INCHES)  OF 
NITROGEN,  PHOSPHORUS  AND  POTASSIUM  1 
*  PRAIRIE  LANDS,  Undulating 


Soil'  Area. 

Soil  Type. 

NITROGEN. 

PHOSPHORUS. 

POTASSIUM. 

Soil. 

Subsoil. 

Soil. 

Sub- 
soil. 

Soil. 

Subsoil. 

Middle  Illinoian  

Brown  silt  loam.  . 
Brown  silt  loam.  . 

4,370 
6,750 

3,440 
3,630 

1,170 
1,410 

2,680 
2,630 

32,240 
45,020 

90,040 
160,140 

Late  Wisconsin  

PRAIRIE  LANDS,  Flat. 


Middle  Illinoian  

Black  clay  loam.  . 

5,410 

3,020 

1,430 

3,030 

31,860 

94,900 

Late  Wisconsin  

Black  clay  loam.  . 

8,900 

3,180 

1,870 

3,090 

37,370 

125,370 

SWAMPS  AND  BOTTOM  LANDS 


Old  bottom  lands  

Deep    gray    silt 
loam 

3,620 

2,280 

1,420 

2,620 

36360 

101  610 

Late  bottom  lands.  .  .  . 
Late  swamp 

Brown  loam  
Deep  peat  

4,720 
34,880 

4,150 
97,730 

1,620 
1,960 

2,410 
3,740 

39,970 
2,930 

119,520 
11  510 

1  Hopkins  and  Pettitt,  Bulletin  No.  123,  University  of  Illinois,  Agricultural  Experiment  Sta- 
tion, 1911. 

As  a  rule  the  proportion  of  organic  matter  in  swamp  soils  increases 
from  the  margin  toward  the  center.  The  marginal  zone  of  soils  catches 
more  of  the  sands,  silts  and  clays  that  are  washed  and  blown  into  the 
swamp  and  in  this  zone  is  found  the  longest  duration  of  rooted  plants 
and  trees.  This  variation  is  well  shown  in  an  analysis  of  soils  from  a 
coastal  plain  swamp  in  North  Carolina  as  follows:1 


Silica. 

Alumina. 

Organic  Matter. 

Water. 

IVlarcinal  soil                  

84.54% 

2.69% 

7  70% 

2  50% 

Swamp  center 

1  52% 

39% 

87  25% 

Q  60% 

Quoted  by  Merrill,  op.cit.  from  Geology  of  North  Carolina,  Vol.  1,  1875. 


258  LAKES  AND  SWAMPS 

River  and  delta  swamps  are  somewhat  subject  to  floods  and  their 
soils  are,  therefore,  likely  to  show  a  higher  content  of  sand,  silt  and 
clay,  but  they  vary  considerably,  so  that  old  river  courses  and  swamps 
may  be  marked  either  by  sandy  loams  or  by  muck  and  peaty  soils, 
Fig.  131.  Muck  and  peaty  soils  have  proved  especially  adapted  to 
onions,  celery  and  other  small  truck  crops.  The  celery  district  of 
Michigan  includes  large  areas  of  cumulose  soils  on  which  the  celery 
is  grown  for  the  most  part.  Many  swamps  in  cool  climates  which  can 
be  inundated  are  used  extensively  for  cranberries.  Swamp  reclamation 
in  many  respects  is  easier  than  lake  reclamation  but,  except  locally, 
but  little  has  been  done  in  North  America.  Shaler  estimates  that  from 
105,200  to  131,200  miles  of  swamps  and  shallow  lakes  in  the  United 
States  can  be  fairly  easily  reclaimed.  As  an  indication  of  the  possibil- 
ities of  these  lands,  the  same  author  estimates  that  "probably  not  far 
from  one-twentieth  of  the  tilled  lands  in  Europe  were  inundated  and 
unfit  for  use  in  the  eighth  century  of  our  era." 

REFERENCES 

Lakes 
G.  K.  GILBERT,  The  Topographic  Features  of  Lake  Shores,  5th  Ann.  Kept.,    U.  S. 

Geological  Survey,  1885,  pages  69-123. 

Lake  Bonneville,  Mon.  1,  U.  S.  Geological  Survey,  1890,  Chapter  2. 
W.  H.  HOBBS,  Earth  Features  and  Their  Meaning,  Macmillan,  1912,  Chapter  29. 
I.  C.  RUSSELL,  Lakes  of  North  America,  Ginn,  1895. 

I.  C.  RUSSELL,  Lake  Lahontan,  Mon.  11,  U.  S.  Geological  Survey,  1885,  Chapter  6. 
R.  D.  SALISBURY,  Physiography,  Holt,  1907,  Chapter  6. 

E.  H.  SELLARDS,  Some  Florida  Lakes  and  Lake  Basins,  6th  Ann.  Rept.,  Fla.  Geolog- 
ical Survey,  1914,  pages  115-159. 
The  Florida  Lakes  and  Lake  Basins,  3d  Ann.  Rept.,  Geological  Survey,  1910,  pages 

47-76. 

TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapter  10. 
WARREN  UPHAM,  The  Glacial  Lake  Agassiz,  Mon.  25,  U.  S.  Geological  Survey,  1896, 
pages  583-591. 

Swamps 

N.  H.  DARTON,  Norfolk  Folio,  U.  S.  Geological  Survey,  1902,  The  Dismal  Swamp 
(A  Coastal  Plain  Swamp). 

A.  W.  GRABAU,  The  Principles  of  Stratigraphy,  Seiler,  1913:  Fresh  Water  Swamps, 
pages  494-509. 

GEORGE  P.  MERRILL,  Rocks,  Rock  Weathering  and  Soils,  Macmillan,  1906,  Chapter 
on  Cumulose  Deposits. 

N.  S.  SHALER,  A  General  Account  of  the  Fresh-water  Morasses  of  the  United  States 
with  a  Description  of  the  Dismal  Swamp  District  of  Virginia  and  North  Caro- 
lina, 10th  Ann.  Rept.,  Part  1,  U.  S.  Geological  Survey,  1888-89. 

Origin  and  Nature  of  Soils,  12th  Ann.  Rept.,  Part  1,  U.  S.  Geological  Survey,  Swamp 
Soils,  pages  311-317. 


CHAPTER  XII 


OCEANS 


Introductory.  —  Oceans  have 
geological  interest  for  several  rea- 
sons: (1)  Most  sedimentary  rocks 
are  of  marine  origin,  and  the  study 
of  ocean  work  helps  in  under- 
standing many  sedimentary  rocks. 
(2)  Recently  upraised  sea  bottoms 
now  constitute  large  and  impor- 


FIG.  244. — Ocean  surf,  Canada.     (Canadian 
Geological  Survey.) 

259 


FIG.   243.— Pebbles   rounded   by   ocean 
waves.     About  one-sixth  natural  size. 


tant  areas  the  world  over. 
(3)  Oceans  are  the  source 
of  most  rainfall  and  (4)  in 
places  they  regulate  tempera- 
tures. 

Movements 

Waves  and  shore  currents 
are  important  geological 
agents  in  oceans  as  well  as 
in  lakes;  shores  are  cut  and 
built  much  the  same  manner 
in  both  bodies  -of  water. 
Tides  are  important  factors 
in  oceans  while  they  are 
practically  negligible  in  lakes. 
Tides  rise  and  fall  twice  a 
day  (twenty-four  hours)  and 
have  two  important  geologi- 
cal effects  on  shore  features. 
In  rising  and  falling,  tides 
widen  the  zone  of  wave  work; 


260 


OCEANS 


for  example,  if  the  range  between  high  and  low  tide  is  5  feet,  the  waves 
can  work  5  feet  higher  than  they  could  without  the  assistance  of  the 
tides.  Then  the  tides  moving  through  narrow  channels  "and  bays 
generate  strong  tidal  currents  that  are  locally  important.  Ocean 
wavfe  erosion  is  more  effective  than  the  wave  work  of  lakes  because  the 
waves  in  oceans  are  higher  and  stronger  and,  furthermore,  wave  work 
has  been  in  operation  longer  in  the  ocean  than  in  lakes,  especially  extinct 
lakes.  A  notable  example  of  wave  erosion  is  the  rapid  destruction  of 
the  island  of  Heligoland  in  the  North  Sea,  which  has  been  almost  entirely 
eroded  since  historical  times.  The  work  of  waves  and  currents  has 
been  discussed  under  the  topic  of  lakes  because  the  resulting  shore 
forms  in  lakes  are  more  important  from  an  agricultural  point  of  view, 
but  it  is  worth  while  to  consider  some  ocean  shore  forms  because  of 
their  wide  extent  and  because  of  their  potential  agricultural  importance. 


Shore  Features 

Barrier  beaches  and  lagoons  are  common  along  low  shelving  coasts 
and  they  extend  with  some  interruptions  along  our  Atlantic  coast  from 

New  York  to  Mexico.  The  action 
of  lake  waves  and  currents  in 
producing  these  features  has 
been  discussed  on  page  242.  The 
processes  involved  when  ocean 
waves  are  concerned  are  practi- 
cally identical  and  need  not  be 
further  discussed.  The  beaches 
are  of  little  agricultural  interest, 
being  for  the  most  part  almost 
pure  sand.  Atlantic  City,  N.  J., 
the  bathing  resort,  and  Galves- 
ton,  Texas,  are  built  on  barrier 
beaches,  Fig.  245.  Between  the 

mainland  and  the  barrier  beach 
FIG.  245,-Barrier  beaches  on  the  Texas     ;g  ft  bod    of  w          either  ^    m 

coast.  .  ,        ,  .  ,  J     ..  / 

brackish,  called  a  lagoon  or  more 

commonly,  a  sound. 

Filling  of  a  lagoon  is  much  the  same  process  as  in  swamps,  except 
that  there  are  often  strong  tidal  currents  in  the  lagoons.  The  lagoon, 
where  shallow,  supports  a  growth  of  rooted  plants;  grasses  and  reeds 


SHORE  FEATURES 


261 


FIG.  246. — Barrier  beaches  and  partly  filled 
lagoons  on  Long  Island,  N.  Y.  (U.  S. 
Bureau  of  Soils.) 


soon  form  a  dense  mat  which  holds  sediments  brought  in  by  tidal  cur- 
rents or  washed  in  from  the  mainland.  As  soon  as  the  water  is  shal- 
lowed so  that  land  is  exposed  at  low  tide,  the  "  salt  grass  "  begins  to 
grow  and  is  sometimes  cut  for  hay.  Meanwhile  sediment  is  blown  into 
the  lagoon  from  the  sandy  barrier  beach,  washed  in  from  the  land  and 
carried  in  by  tidal  currents 
through  openings  in  the 
barrier  beach.  Mussels  and 
other  marine  animals  may 
also  contribute  to  the  accu- 
mulating materials.  Thus 
as  a  result  of  lagoon  filling 
there  is  often  a  belt  of  low- 
lying  level  land  which  fre- 
quently contains  partially 

decayed  plant  roots  and  is  sometimes  underlain  by  peat.  The  soil  texture 
is  usually  somewhat  heavy,  although  the  soil  near  the  barrier  beach  is 
often  sandy  because  of  the  sand  that  is  blown  or  washed  in.  The  soils 
near  the  mainland  are  also  lighter  in  texture  and  are  somewhat  higher, 
all  because  of  the  in  wash  from  the  mainland.  These  marshes  are 

sometimes  drained  and  in 
cultivation,  but  reclamation 
of  ocean  marshes  in  North 
America  has  hardly  begun, 
although  large  areas  have 
been  brought  under  culti- 
vation in  Europe.  These 
marshes  in  North  America 
are  a  valuable  reserve  of 
FIG.  247.— Reclaimed  tidal  flats,  Cal.  (U.  S.  arable  land  when  economic 
Bureau  of  Soils.)  conditions  favor  their  recla- 

mation.      Extensive    diking 

is  necessary,  ditching  is  required  and  the  soils  must  often  be  exposed 
for  a  considerable  time  for  the  sea  salt  to  be  washed  out.  Salt  marsh 
and  fresh- water  swamps  often  meet  in  estuaries,  the  marshes  extending 
as  far  inland  as  the  tides  penetrate. 


262 


OCEANS 


REFERENCES 
Marine  Marshes 

A.  W.  GRABAU,  The  Principles  of  Stratigraphy,  Seiler,  1913:  Marine  Marshes,  pages 

487^94. 
N.  S.  SHALER,  Sea-coast  Swamps  of  the  Eastern  United  States,  6th  Ann.  Rept.,U.  S. 

Geological  Survey,  1885. 
Beaches  and  Tidal  Marshes  of  the  Atlantic  Coast  in  Physiography  of  the  United 

States,  American  Book  Co.,  1896. 
Origin  and  Nature  of  Soils,  12th  Ann.  Kept.,  Part  1,  U.  S.  Geological  Survey,  Marine 

Marshes,  pages  317-23. 

Sea  Islands. — An  interesting  and  important  feature  off  the  shore 
of  North  Carolina  and  Georgia  is  the  "  Sea  Islands  "  which  are  well 

known  for  their  production  of 
sea-island  cotton,  Fig.  248. 
The  islands  are  separated  by 
narrow  tidal  streams  and 
marshes  and  are  apparently  the 
joint  result  of  waves,  currents 
and  of  scour  by  the  tides,  all  of 
which  agents  have  worked  over 
the  sediments  brought  by  the 


MARSH         --"—•= 

BARRIER  BEACH  SANDS  Jv.v.'.vX 


FIG.  248.— "Sea  islands,"  S.  C. 


rivers.  The  sea  islands  are 
practically  duplicated  in  many 
shore  regions  along  the  Atlantic 

coast.     The  arable  portions  are  largely  of  sandy  materials  and  each 

island  is  surrounded  by  a  belt  of  muck. 


Depressed  and  Elevated  Coasts 

Introductory. — To  ordinary  observation,  coasts  appear  stationary, 
but  geological  observations  show  that  most  of  our  coasts  have  been 
either  elevated  or  depressed  within  comparatively  recent  geological 
time.  These  movements,  however,  are  so  slow  that  only  in  exceptional 
cases  of  very  rapid  movement  can  they  be  detected  as  occurring  within 
historical  time.  Moreover,  the  coastal  movements  are  seldom  simple,  a 
general  elevation,  for  example,  often  being  interrupted  at  various  times 
by  sinking.  It  must  also  be  remembered  that,  when  one  speaks  of  a 
rising  or  sinking  coast,  the  movement  may  affect  large  areas  both  above 
and  below  the  water,  but  the  movements  are  more  easily  recognized  at 
the  coast. 


DEPRESSED  AND  ELEVATED  COASTS 


263 


Depressed  Coasts. — When  the  land  adjoining  a  coast  sinks  or  the 
ocean  rises,  the  former  coast  will  be  submerged.  Hills  and  mountains 
will  be  changed  to  islands.  The  ocean  waters  will  extend  far  up  the 
valleys  as  salt  or  brackish  water  embay ments  and  the  former  valleys 
may  often  be  traced  by  soundings  for  considerable  distances  beneath 
the  ocean.  Such  valleys  are  appropriately  termed  "  drowned  valleys." 
Most  of  the  best  harbors  of  the  world  are  due  to  the  drowning  of  valleys, 
examples  of  which  are  those  of  New  York,  Philadelphia,  Baltimore, 
Liverpool  and  London.  The  Hudson  Fi/er,  for  example,  formerly 
entered  the  Atlantic  over  100 
miles  southeast  of  its  present 
mouth.  A  considerable  de- 
pression completely  submerged 
the  lower  part  of  the  valley 
and  drowned  the  present  valley 
so  that  tidal  effects  extend 
nearly  to  Albany,  more  than 
100  miles  from  New  York, 
and  the  former  deep,  canyon- 
like  valley  has  been  filled 
with  thick  sediments.  The 
submerged  Hudson  Valley  is 
shown  in  Fig.  249.  FIG.  249.— The  submerged  (drowned)  lower 

It  is,  therefore,  evident  Hudson  Valley.  The  figures  show  depths 
that  a  strongly  irregular  coast,  in  fathoms.  (U.  S.  Coast  and  Geodetic 
deeply  notched  by  estuaries,  is  urvey.) 

an  indication  of  submergence; 

a  possible  exception  is  a  fiord  coast  (page  221),  where  valleys  have 
been  deepened  by  glaciers,  but  even  many  of  these  coasts  are  known 
to  have  been  sunken.  The  Atlantic  coast  of  the  United  States  is  in 
general  a  sunken  coast  while  much  of  the  Pacific  coast  is  a  risen 
coast;  the  contrast  between  the  deeply  indented  Atlantic  coast  and 
the  relatively  smooth  Pacific  coast  is  at  once  evident  from  a  glance  at 
a  map.  Evidently  the  sinking  of  coasts  submerges  lands  that  might 
be  arable  but,  on  the  other  hand,  the  process  furnishes  harbors  and 
facilities  for  commerce  and  the  waters  of  the  estuaries  temper  the 
climate  of  adjoining  regions. 

Elevated  Coasts. — From  an  agricultural  point  of  view  the  rising  of 
coasts  is  extremely  important  since  such  a  movement  adds  to  the  land 
area  of  a  continent.  The  uplifted  sea  bottom  is  usually  smooth  because 


264 


OCEANS 


(1)  it  has  been  overspread  with  sediments  which  tend  to  make  a  smooth 
surface  and  (2)  it  has  been  but  little  eroded.  Consequently  the  meeting 
of  land  and  sea  on  the  smooth  surface  of  the  uplifted  sea  bottom  is 
relatively  smooth  and  risen  coasts  are  usually  but  little  indented. 
Exceptionally,  a  rather  irregular  sea  bottom  may  be  uplifted  and  pro- 
duce an  irregular  shore  line  but  such  are  infrequent.  Uplift  of  coasts  is 
also  indicated  by  cliffs,  caves,  beaches  and  other  shore  features  now 
found  in  many  places  above  the  present  water  level,  the  distances  above 


EMERGED  COASTAL  PLAIN 

INNER  PLAIN  OUTER  PLAIN 


SUBMERGED 
COASTAL  PLAIN 


FIG.  250. — The  submerged  coastal  plain  and  the  inner  and  outer  parts  of  the  emerged 

coastal  plain 

water  level  varying  from  a  few  feet,  showing  a  slight  uplift,  to  shore 
features  scores  and  hundreds  of  feet  above  the  present  sea  level. 


Coastal  Plains 

Coastal  plains,  as  the  term  implies,  are  plains  along  the  coast,  plains 
varying  in  width  from  a  few  miles  to  those  scores  of  miles  in  width. 
Some  narrow  coastal  plains  are  due  to  a  slight  sinking  which  has  sub- 
merged a  wider  coastal  plain,  others  are  due  to  infilling  of  sediment 
from  the  mainland,  but  the  great  coastal  plains  of  the  world  are  due  to 
extensive  uplifting  which  exposes  large  areas  that  were  formerly  sea 
bottoms. 


THE  COASTAL  PLAIN  OF  NORTH  AMERICA 


265 


The  Coastal  Plain  of  North  America 

This  is  so  extensive  and  important  that  it  merits  a  somewhat 
thorough  consideration.  It  may  be  said  to  begin  at  New  York  and 
with  some  interruptions  to  extend  to  Yucatan.  At  first  a  narrow  strip 
in  New  Jersey,  the  Coastal  Plain  widens  to  the  southward  and  attains 
a  width  of  over  150  miles  in  Alabama  from  whence  it  again  narrows 
through  Texas,  the  whole  resembling  a  vast  crescent,  interrupted  by 
the  Mississippi  and  other  low- 
lands, Fig.  252.  This  is  one 
of  the  most  persistent  and 
important  features  in  North 
America  and  it  includes  mil- 
lions of  acres  of  arable  land. 

Origin. — The  Coastal  Plain 
is  essentially  an  upraised  sea 
bottom  much  of  which  has 
been  but  a  comparatively 
short  time  above  water  and, 
furthermore,  soundings  off 
the  Atlantic  and  Gulf  coasts  FIG.  252 -Map  showing  the  general  distribu- 
.  ,  .  ,  .  ,  tion  of  the  Lafayette  and  Columbia  forma- 

shows  that  this  plain  extends        tiong     (After  McGee>  glightly  modified-) 

beneath  the  ocean  with  no  The  insert  map  shows  the  Coastal  Plain, 
notable  change  or  interrup- 
tion at  the  present  shore,  Fig.  250.  It  will  be  remembered  that 
the  lower  Hudson  Valley  has  been  drowned  and  other  rivers  flow- 
ing over  the  Coastal  Plain  show  submerged  lower  valleys,  and  from  this 
and  other  facts  it  is  clear  that  much  of  the  former  Coastal  Plain  is  now 
submerged.  In  other  words,  the  Coastal  Plain  was  formerly  above 
water,  at  least  long  enough  for  rivers  to  cut  deep  valleys  in  its  surface, 
and  then  much  of  the  plain  sank  so  that  the  Atlantic  and  Gulf  meets 
the  land  along  the  present  indented  coast ;  the  Coastal  Plain,  therefore, 
consists  of  two  parts,  the  emerged  and  the  submerged. 

The  materials  of  the  Coastal  Plain  are  derived  from  sediments  which 
were  washed  into  the  sea  from  the  lands  and  from  the  skeletons,  shells 
and  other  remains  of  animals  which  have  accumulated  on  the  ocean 
bottom;  in  places  there  are  deposits  of  lignite  due  mostly  to  the  par- 
tial decay  of  plants.  Typically  the  sediments  are  unconsolidated  and 
consist  mostly  of  sands  and  clays  although  locally  there  are  sandstones, 
limestones  and  shales.  Most  of  these  materials  are  stratified  and  in 


266  OCEANS 

general  the  strata  dip  gently  seaward  toward  the  Atlantic  and  the  Gulf 
of  Mexico.  The  low  dip  brings  to  the  surface  the  edges  of  the  strata 
as  heretofore  explained,  Fig.  51,  so  that  the  various  formations  come 
to  the  surface  in  belts  that  are  roughly  parallel  to  the  coast. 

Boundaries. — Thus,  in  passing  across  the  Coastal  Plain  from  the 
shore  inland,  one  will  cross  successive  belts,  each  older  than  its  shore- 
ward neighbor.  Finally  at  the  edge  of  the  Coastal  Plain  there  are  older 
rocks  from  which  the  sediments  of  which  the  plain  is  composed  were 
derived,  the  sediments  being  washed  down  from  these  rocks  into  the 
ocean  which  then  covered  the  Coastal  Plain.  From  New  Jersey  to 
central  Alabama  these  older  rocks  are  mostly  crystalline,  they  rise 
above  the  plain  and  the  feature  in  general  is  termed  the  Piedmont 

Plateau.  From  Alabama  to 
Texas  these  old  rocks  are 
largely  sedimentary.  As 
streams  cross  from  the  harder 
older  rock  to  the  weaker 
rocks  of  the  Coastal  Plain, 
there  are  usually  found 
rapids  or  falls  near  the  junc- 
tion of  the  older  and  younger 

FIG.  253.— The  Coastal  Plain,  Va.     (U.  S.          rocks  and  hence  this  bound- 
Geological  Survey.)  ary  of   the    Coastal  Plain  is 

sometimes    called    the    Fall 

Line  although  "  fall  zone  "  would  be  a  better  term.  There  is  seldom 
a  sharp  topographic  boundary  between  the  rocks  of  the  old  land  and 
those  of  the  Coastal  Plain  except  in  some  parts  of  Texas  where  faulting 
has  produced  steep  slopes  between  the  two  divisions,  Fig.  62. 

Erosion  of  the  Coastal  Plain. — This  uplifted  sea  bottom  was  at  first 
smooth  and,  indeed,  much  of  the  area  now  has  such  a  gentle  slope  sea- 
ward that  it  appears  flat  to  the  eye  and  so  gentle  are  the  slopes  that 
much  of  the  rainfall  sinks  into  the  soil  instead  of  running  off.  The 
erosion,  however,  in  all  parts  has  not  been  equal.  The  regions  next 
to  the  older  rocks  have  been  longer  exposed;  they  are,  in  general,  higher 
than  the  regions  nearer  the  ocean  and  are,  therefore,  more  hilly.  The 
seaward  region,  on  the  other  hand,  has  been  exposed  to  erosion  for  a 
comparatively  short  time,  it  is  lower  and,  in  consequence,  the  surface 
is  but  little  eroded  and  many  areas  are  substantially  the  same  as 
when  they  emerged  from  the  waters.  Thus,  in  most  parts  of  the  Coastal 
Plain,  there  are  two  distinctive  belts  although  the  boundaries  are  indef- 


THE  COASTAL  PLAIN  OF  NORTH  AMERICA 


267 


inite, — the  inner  (higher  and  older)  belt,  often  known  locally  as 
"  The  Hills/'  and  the  outer  and  lower  belts,  commonly  termed  "  The 
Flats."  These  features  are  illustrated  in  Fig.  250,  which  shows  the  sub- 
merged portion  known  here  as  the  Continental  Shelf  and  the  emerged 
portion  with  its  smooth  and  eroded  portions. 

As  the  Coastal  Plain  is  eroded,  the  more  resistant  formations  will 
stand  out  as  ridges  and  the  less  resistant  ones  will  be  more  rapidly 
reduced  to  lowlands.  The  ridges  of  the  resistant  strata  will  typically 
have  a  gentle  slope  seaward  with  the  dip  of  the  rocks  and  a  steeper 
slope  to  the  landward;  such  a  ridge  is  termed  a  cuesta.  Fig.  251  shows  a 
cuesta  and  a  lowland  in  a  coastal  plain,  the  cuesta  being  formed  by  a 


CHENNUGA 
RIDGE 


FIG.  251. — A  part  of  the  Coastal  Plain  in  Alabama. 

somewhat  resistant  formation  and  the  lowland  by  the  erosion  of  a 
weak  chalky  limestone.  The  diagram  shows  Chennuga  Ridge,  one  of 
the  largest  cuestas  in  America.  This  ridge  rises  gradually  from  the 
lowlands  on  the  south  and  descends  rather  abruptly  to  the  Black  Belt 
on  the  north,  so  called  from  its  black  soils.  These  features  extend  more 
or  less  parallel  to  each  other  across  Georgia,  Alabama  and  Mississippi. 
It  often  happens  that  formations  making  cuestas  and  lowlands  change 
their  character  so  that  these  features  are  seldom  continuous  for  very 
great  distances. 

The  Lafayette  and  Columbia  Formations. — While  the  formations 
of  the  Coastal  Plain  come  to  the  surface  in  roughly  parallel  belts,  the 
residual  soils  often  show  very  little  of  this  belted  arrangement  and 
frequently  they  are  entirely  unrelated  to  the  underlying  formations. 
Sandy  soils,  for  example,  are  often  found  overlying  heavy  clays,  clays 
which  would  yield  heavy  soils  such  as  clay  loams.  The  explanation 
of  this  is  due  to  the  fact  that  there  is  a  thin  mantle,  typically  sandy, 


268  OCEANS 

which  overlies  the  formations  of  much  of  the  Coastal  Plain.  The  soils 
from  this  formation  are  often  unlike  those  from  an  adjacent  underlying 
formation  so  that,  in  the  Coastal  Plain  as  a  whole,  the  soils  do  not  show 
the  belted  arrangement  that  might  be  expected  from  the  belted  arrange- 
ment of  the  rock  outcrops.  This  relatively  thin  blanket,  typically  of 
sandy  and  gravelly  materials,  belongs  to  the  Lafayette  and  Columbia 
formations.  These  formations  are  among  the  most  important  soil 
formers  in  North  America. 

Of  these  formations,  the  Lafayette  is  the  older  and  most  extensive, 
the  Columbia  always  overlying  the  Lafayette  where  they  are  found 
together.  They  are  typically  sandy  and  in  places  gravelly,  although 
often  there  are  considerable  areas  of  clays  and  silts;  the  similarity  of 
these  two  formations  often  makes  it  very  difficult  to  distinguish  them. 
In  color  they  are  reddish  or  yellowish,  as  is  attested  by  such  local  names 
as  u  orange  sand,"  "  red  hills/'  etc.  The  thickness  varies  greatly  some- 
times in  short  distances ;  thicknesses  of  over  a  hundred  feet  have  been 
observed,  while  in  other  places  the  formations  may  be  absent  or  only  a 
few  inches  thick,  and  probably  the  formations  as  a  whole  are  under 
thirty  feet  in  thickness.  It  must  be  remembered  that  while  in  Fig.  252 
the  general  occurrences  are  shown,  they  are  not  to  be  found  over  the 
entire  areas;  they  are  absent  over  considerable  areas  and,  moreover,  as 
the  soils  are  studied  more  thoroughly  it  is  becoming  apparent  that  these 
formations  are  so  thin  in  many  places  that  they  influence  many  soils 
but  slightly.  In  composition  and  structure,  both  formations,  especially 
the  Lafayette,  change  quickly  from  place  to  place  and  both  frequently 
show  irregular  stratification;  roughly  stratified  sands  and  gravels  may 
be  found  in  one  place  which  may  change  in  a  few  rods  to  sands  or  to 
unstratified  sandy  clays. 

Origin  of  the  Lafayette  and  Columbia  Formations. — Three  signif- 
icant facts  are  clear.  The  frequent  stratification  and  rounded  gravels 
indicate  unquestionably  that  the  formations  were  laid  down  by  running 
water;  the  fossils  often  to  be  found  in  the  cherty  pebbles  are  from  older 
formations  which  are  often  far  to  the  northward;  the  reddish  colors 
and  the  insoluble  characters  of  the  gravels,  sands,  silts  and  clays  point 
to  long-continued  weathering  of  the  rocks  from  which  the  Lafayette 
and  Columbia  were  derived.  Two  views  of  their  deposition  have  been 
set  forth.  According  to  one  view,  the  formations  were  deposited  in  the 
ocean  as  the  Gulf  and  Atlantic  were  retreating  toward  their  present 
positions.  Another  view,  which  at  present  seems  best  to  explain  the 
facts,  is  that  much  of  these  materials  were  laid  down  by  streams  flowing 


MARINE  EEIOSITS  269 

over  the  lands,  in  short  that  the  materials  are  river  deposited  (fluvial) 
rather  than  marine.  The  question  is  still  an  open  one,  with  the  pos- 
sibility that  both  processes  may  have  been  involved. 

REFERENCES 

W.  J.  McGEE,  The  Lafayette  Formation,  12th  Ann.  Kept.,  U.  S.  Geological  Survey, 
1891,  pages  360-380. 

CHAMBERLIN  and  SALISBURY,  Geology,  Vol.  3,  Holt,  1907,  The  Lafayette  Forma- 
tion, pages  301-308. 

H.  H.  BENNETT,  Soils  of  the  Atlantic  and  Gulf  Coastal  Plain  Province  in  Bull.  96, 
U.  S,  Bureau  of  Soils,  1913;  General,  pages  221-229,  Soil  Series,  pages  229-301. 

N.  S.  SHALER,  Origin  and  Nature  of  Soils,  12th  Ann.  Kept.,  Part  1,  U.  S.  Geological 
Survey,  1890-91;  Soils  of  Newly  Elevated  Sea  Bottoms,  pages  245-250. 

Marine  Deposits 

Marine  deposits  may  for  convenience  be  divided  into  two  classes: 
(1)  those  carried  from  land  and  (2)  those  derived  from  the  water  itself 
and  its  life.  The  land  sediments  are  for  the  most  part  deposited  in  rel- 
atively shallow  water;  they  vary  quickly  as  the  water  deepens  so  that 
there  are  shore  (littoral)  deposits  which  extend  between  high  and  low- 
tide  levels  and  shoal  water  deposits  which  extend  approximately  to 
depths  of  600  feet.  The  shore  deposits  naturally  are  closely  influenced 
by  the  shore  materials,  being  sandy,  for  example,  where  the  shores  are 
sandy;  in  general,  these  deposits  are  somewhat  coarse.  As  deeper 
waters  are  reached,  the  influences  of  the  waves,  currents  and  rivers 
become  less  and  only  finer  materials  are  deposited.  Furthermore,  in 
these  deeper  waters  calcareous  materials  from  the  shells  and  skeletons 
of  animals  are  added  to  the  muds  and  sands  derived  from  the  lands. 
Thus  in  some  formations,  long  since  changed  into  solid  rock,  there  are 
fine  sandstones  and  conglomerates  grading  laterally  into  shales  and 
these  in  turn  into  limestones,  the  entire  formation  representing  shore 
deposition,  off-shore  deposition  and  finally  deeper  water  deposition. 

Deep  Water  Deposits. — The  deeper  waters  at  a  considerable  dis- 
tance from  shore  contain  but  little  land  material.  From  about  1,000 
to  15,000  feet  the  ocean  floor  is  mostly  covered  with  a  bluish-gray  mud 
with  some  shell  fragments  and  frequently  intermingled  with  minute 
shells.  These  shells,  when  numerous,  have  formed  beds  of  chalk. 
The  minute  animals  inhabiting  the  shells  live  near  the  surface  of  the 
water  and,  as  the  animals  die,  the  shells  shower  down  on  the  ocean 
bottom.  In  the  deep  or  abysmal  waters  the  ocean  bottom  is  covered 


270  OCEANS 

with  a  reddish  clay  made  by  the  extremely  slow  accumulation  of  vol- 
canic fragments,  by  minute  meteorites  and  by  the  insoluble  materials 
of  shells  and  skeletons  which  have  been  dissolved  during  their  slow 
sinking.  It  is  interesting  in  this  connection  to  note  that,  with  a  few 
possible  minor  exceptions,  the  deposits  in  very  deep  water  do  not  seem 
to  have  any  counterparts  in  our  stratified  rocks.  This  indicates  that, 
so  far  as  our  observations  show,  these  deposits  have  seldom,  if  ever,  been 
raised  to  form  continents  or,  in  other  words,  the  continents  and  present 
oceans  have  always  or  for  a  very  long  time  maintained  their  present 
locations.  Chemical  precipitates  in  the  ocean  are  rare,  for  the  water  in 
the  open  ocean  can  rarely  become  saturated  with  any  soluble  materials, 
salt,  for  example.  On  the  other  hand,  alterations  by  which  soluble 
materials  are  converted  to  insoluble  compounds  and  are  thereby  pre- 
cipitated are  not  uncommon.  A  case  in  point  is  the  greenish  mineral, 
glauconite,  which  is  a  compound  of  iron,  potash,  silicon  and  oxygen. 
This  has  been  found  in  process  of  formation  in  shallow  seas  and  appears 
to  originate  through  the  interaction  of  decaying  animals  and  the  sea- 
bottom  muds.  Glauconite  has  been  used  to  a  limited  extent  as  a  potash 
fertilizer. 

Sea  life  is  an  important  geological  agent.  Most  of  the  sea  life, 
both  plants  and  animals,  lives  in  the  more  shallow  waters,  but  the  sur- 
face waters  of  the  entire  ocean,  especially  in  the  warmer  portions,  teem 
with  marine  life.  The  shells  and  skeletons  sink  to  the  bottom  in  rela- 
tively shallow  waters  while  in  deep  waters  these  materials  are  wholly  or 
partially  dissolved  during  the  long  sinking. 

Corals  are  simple  animals  (polyps),  some  species  of  which  secrete 
lime  carbonate  from  the  water  and  build  it  into  their  bodies.  As  the 
animal  dies  the  skeleton  remains  and,  since  most  corals  live  in  colonies, 
their  innumerable  skeletons  form  large  masses,  a  coral  reef,  for  example, 
near  Australia  being  1,000  miles  or  more  in  length.  These  skeletons 
become  solidified  by  redissolving  and  depositing  and  this  process  is 
aided  by  animals  which  bore  through  the  coral  masses.  The  coral 
masses  are  broken  into  coral  sand  by  some  animals  which  bore  into  them 
and  especially  are  they  broken  up  by  the  waves  which  pound  them  into 
coral  sand,  which  may  be  strewn  over  the  ocean  bottom.  Building 
species  of  corals  live  only  in  warm  water  (60°  F.  or  more)  and  in  shallow 
waters  of  not  over  150  feet  depth.  They  flourish  best  in  moving  water 
free  from  sediment.  Coral  skeletons  have  formed  enormous  masses 
of  limestone  rock,  some  of  which  shows  the  fossil  forms  in  great  perfec- 
tion, Fig.  254. 


MARINE  DEPOSITS 


271 


FIG.  254. — Ancient  and  extinct  coral  at 
left;  modern  coral  at  the  right. 


the 


A  great  variety  of  other  animals  such  as  oysters,  clams,  etc.,  and 
also  minute  animals  have  contributed  their  shells  to  ocean  mud  which 
has  often  been  changed  into 
rock.  An  indication  of  the 
work  of  animals  and  plants  in 
abstracting  lime  and  mag- 
nesia from  the  sea  water  is 
seen  in  the  fact  that,  while 
lime  and  magnesia  constitute 
over  half  the  solution  load  of 
rivers,  the  ocean  waters,  on 
the  other  hand,  contain  scarce- 
ly a  twentieth  of  these  ele- 
ments, the  reason  being,  of 
course,  that  they  are  used  by 
sea  life  in  skeletons  and  shells. 
Kelp,  a  large  sea  weed,  has 

agricultural  interest  because  this  plant  secretes  considerable  potash, 
which  is  now  being  extracted  on  a  commercial  scale. 

It  must  not  be  forgotten  in  this  connection  that  sedimentary  rocks 

of  considerable  importance  have 
accumulated  in  other  bodies  of 
water  than  oceans.  Many  of 
these  rocks  were  accumulated 
in  extinct  lakes  and  even  in 
rivers.  Indeed  there  are  exten- 
sive formations,  largely  of  sand- 
stone and  conglomerate,  which 
are  believed  to  have  been  depos- 
ited on  land.  A  case  in  point 

are    the    alluvial    materials    which    were    washed    from    the    Rocky 
Mountains  and  strewn  over  the  plains  to  the  eastward,  page  174. 


FIG.  255. — Coquina  limestone,  a  mass  of 
cemented  shells  at  the  left;  coralline 
limestone  in  the  center;  massive  lime- 
stone at  the  right. 


REFERENCES 

CHAMBERLIN  and  SALISBURY,  Geology,  Holt,  1904,  Vol.  1,  Chapter  6. 

JAMES  GEIKIE,  Earth  Sculpture,  Putnam,  1898,  Chapter  15. 

G.  K.  GILBERT,  Topographic  Features  of  Lake  Shores,  Fifth  Ann.  Kept.  TL  S. 

Geological  Survey,  1885. 
W.  H.  HOBBS,  Earth  Features  and  Their  Meaning,  Macmillan,  Chapters  18  and  19. 


272  OCEANS 

DOUGLAS  WILSON  JOHNSON,  Shore  Processes  and  Shoreline  Development,  John 

Wiley  &  Sons,  1919. 

RIES  and  WATSON,  Engineering  Geology,  Wiley  &  Sons,  1914,  Chapter  8. 
N.  S.  SHALER,  The  Geological  History  of  Harbors,  13th  Ann.  Kept.  U.  S.  Geological 

Survey,  1893. 

TARR  and  MARTIN,  College  Physiography,  Macmillan,  1914,  Chapter  11. 
THOMAS  and  WATT,   Improvement  of  Rivers,  2  vols.,  2d  Edition,  Wiley  &  Sons, 

1913,     (See  especially  for  Harbors.) 


CHAPTER  XIII 
MINERAL  FERTILIZERS 


PHOSPHATES 

Kinds. — Phosphates  are  combinations  of  phosphoric  acid  with  some 
base  like  lime  or  iron.  There  are  many  different  varieties  of  phosphates, 
but  those  used  for  fertilizing  purposes  are  confined  to  the  phosphates  of 
lime,  of  which  apatite  is  the  crystalline  form  and  phosphorite  is  the  non- 
crystalline,  amorphous  and  usually  impure  phosphate  of  commerce. 

Apatite  (Ca5(ClF)(PO4)3  (page  8)  is  a  very  common  though  not 
extensive  mineral  of  igneous  and  metamorphic  rocks,  and  in  all  probabil- 
ity these  rocks  are  the  primary  source  of  commercial  phosphates. 
Apatite  is  very  seldom  found  in  sufficient  quantity  to  admit  of  direct 
production,  but  in  a  few  instances  it  is  produced  in  small  quantities  as 
a  by-product  in  some  mining  operations.  Phosphorite  is  an  inclusive 
term  covering  the  more  or  less  impure  and  amorphous  deposits  of  lime 
phosphates. 

Phosphate-bearing  Rocks. — Fig.  256  shows  the  phosphoric  acid 
content  of  various  rocks.  It  should  be  remembered,  however,  that, 

0,50$ 

BASALT  0. 48 

DIORITE          0..26 
GRANITE         0.24^ 
LIMESTONE  0.23 
SHALE  0.15^ 

SANDSTONE  0.06^ 

FIG.  256. — Diagram  showing  a  comparison  of  phosphoric  acid,  P2O6  (black  lines)  in 
different  rocks.     (Data  after  Stokes  and  Daly.) 

owing  to  the  relatively  small  number  of  analyses,  the  conclusions  are 
only  tentative  and  may  not  hold  indefinitely.  The  igneous  rocks  have 
a  relatively  high  phosphoric  acid  content  almost  entirely  in  the  form  of 
apatite,  the  lime  phosphate.  The  ferro-magnesian  rocks  show  a  high 
content.  The  limestones  easily  lead  among  the  sedimentary  rocks 

273 


13?                                           Ut 

r— 

• 

274  MINERAL  FERTILIZERS 

and  in  this  connection  it  will  be  remembered  that  phosphatic  limestones 
are  the  chief  source  of  phosphoric  acid  for  commercial  purposes.  A 
fair  amount  is  found  in  shales,  while  sandstones  show  the  lowest  content. 
General  Origin. — Like  most  mineral  deposits,  the  commercial  phos- 
phates have  been  concentrated  from  large  bodies  of  low-grade  phosphate 
rock  into  more  or  less  impure  phosphates  by  some  of  the  following 
processes:  (1)  In  some  cases  the  phosphates  have  been  leached  out  of 
low-grade  phosphatic  limestones  by  underground  waters,  and  these 
waters  have  carried  away  the  phosphates  and  deposited  them  in  some 
other  formation.  (2)  Another  form  of  leaching  and  deposition  is  seen 
in  the  unconsolidated  materials  of  the  Coastal  Plain  where  the  phos- 
phatic materials  of  bones  and  shells  are  dissolved,  carried  away  and 
deposited  elsewhere.  (3)  Leaching,  acting  in  a  somewhat  different 
way,  also  has  produced  phosphate  deposits.  Phosphate  nodules  or 

pebbles    are     often     scattered 
through  a  phosphatic  limestone; 
the  underground  waters  dissolve 
FiG7257.-Nodules  of  lime  phosphate,  one-      the    enclosing  limestone   faster 
sixth  natural  size.    The  phosphate  was      than  they  dissolve  the  nodules, 
carried    in    solution    and    deposited    as      so  that  the  phosphatic  nodules 
nodules.  are    concentrated    by    the    re- 

moval of  the  enclosing  matrix. 

(4)  Again  the  ground  waters  sometimes  carry  phosphatic  materials  in 
solution  and  replace  other  minerals  such  as  calcite,  so  that  the  lime- 
stone becomes  enriched;  for  example,  in  some  of  the  Tennessee  phos- 
phates, the  ground  waters  have  carried  phosphatic  materials  into  a 
cherty  limestone,  dissolving  the  calcite  and  replacing  it  with  phosphatic 
materials  and,  as  a  result,  the  fragments  of  chert  are  found  imbedded 
in  phosphatic  materials,  thus  forming  a  coarse  breccia. 

The  primary  origin  of  phosphates  is  but  little  understood,  but  prob- 
ably, as  has  been  noted,  they  were  originally  in  the  form  of  apatite;  the 
phosphates  have  often  undergone  many  changes  before  assuming  their 
present  position;  "  they  may  exhibit  considerable  coplexity  of  origin, 
involving  in  some  cases  several  shifts  of  the  phosphatic  material " 
(Ries).  The  original  apatite  of  the  rocks  was  slowly  dissolved  in  water 
and  carried  to  the  seas  from  which  it  was  assimilated  into  the  bones, 
shells  and  tissues  of  sea  animals.  As  the  animals  died  their  remains 
settled  to  the  sea  bottom  where  they  were  subjected  to  the  leaching  and 
concentration  which  have  been  described  and  when  conditions  were 
especially  favorable,  accumulations  of  phosphorites  resulted.  In  a 


THE  TENNESSEE  PHOSPHATES 


275 


sense  phosphatic  limestones  are  impure  limestones,  the  lime  phos- 
phate along  with  iron  oxides,  clay  and  other  materials  being  more  or 
less  incidental.  Nearly  all  sedimentary  rocks  contain  some  phos- 
phate as  is  shown  by  Fig.  256.  As  a  rule  phosphatic  rocks  do  not  show 
any  special  characteristics  whereby  they  may  be  readily  recognized 
and  they  are  seldom  discovered  except  by  chemical  tests.  There  is, 
therefore,  reason  to  believe  that  in  all  probability  there  are  many 
undiscovered  phosphatic  rocks  in  North  America. 

(5)  Finally,  streams  have  eroded  phosphatic  formations  and  con- 
centrated the  nodules  in  the  stream  deposits,  a  type  well  shown  in 
Florida. 

PHOSPHATE  PRODUCING  REGIONS 

The  United  States  is  by  far  the  leading  producer  of  rock  phosphate 
and  enormous  reserves  are  as  yet  practically  untouched.  Rock  phos- 
phate is  one  of  our  principal  exports  to  Europe.  Phosphate  is  a  very 
important  factor  in  the  production  of  the  world's  food  supply.  At 
present  the  principal  phosphate-producing  fields  in  North  America 
are  in  Tennessee  and  Florida. 


The  Tennessee  Phosphates 

These  phosphates  are  found  in  five  ancient  formations  of  Ordovician 
and  Devonian  ages  (page  294).  The  phosphatic  strata  are  by  no  means 
continuous  and  a  single  stratum  may  vary  greatly  in  thickness  and  in 
phosphate  content.  Residual  phosphate. — By  far  the  most  important 
commercial  products  of  these  deposits  is  the  "  brown  phosphate  "  which 
is  mostly  derived  from  Ordovician  limestones  and  is  due  to  leaching. 

The  brown  phosphate  is  the 
residual  material  left  after 
the  phosphatic  limestone 
has  been  weathered  and 
leached.  This  process  is 
illustrated  in  Fig.  258,  where 


FIG.  258. — Diagram  showing  the  occurrence  of 
brown  phosphate  in  Tennessee.  (After 
Hayes,  U.  S.  Geological  Survey.) 


stratum  B  represents  a 
phosphatic  limestone.  The 
ground  water,  entering  the 

rock  through  joints  and  other  openings,  dissolves  the  soluble 
materials;  the  calcite  and  dolomite  are  more  soluble  than  the 
phosphatic  materials  so  that  the  latter  are  left  as  residual  material 


276 


MINERAL  FERTILIZERS 


along  with  varying  amounts  of  clay,  iron  oxides  and  sand.  Solu- 
msfr  _____ tion  work  is  naturally  un- 

equal and  portions  of  the 
rock  remain  intact,  the  so- 
called  "  horses"  (H)  of  the 
miners  (Fig.  258).  The  strata 
typically  settle  unequally, 
forming  the  "  wavy  "  struc- 
ture often  seen  in  the  pits 
and  which  is  shown  in  the 
figure.  Brown  phosphate  is 
taken  from  open  surface  pits. 

FIG.  259.-Limestone  "horses"  in  brown  phos-  r  The  rocks  ^^^  the 
phate,  Tenn.  (Phalen,  U.  S.  Geological  Tennessee  brown  residual 
Survey.)  phosphates  are  practically  all 

limestones,  from   2   to   8  feet 

thick,  which  contain  large  amounts  of  fish  egg-like  (oolitic)  grains. 
These  grains  are  believed  by  Hayes  to  be  fragments  of  shells  and  bones 
deposited  in  rather 
shallow  water  and 
rounded  by  wave 
and  current  action.1 
Very  probably  these 
grains  have  been 
enlarged  by  coat- 
ings of  phosphorite 
which  were  depos- 
ited from  the  water, 
Fig.  260. 

Bedded  Rock  Phos- 
phates. —  The  less 
important  type  of 
Tennessee  phos- 
phorite is  found 

in  Devonian  rocks  -r,      0™     ,,.       , 

u.  ,  FIG.  260.— Microphotograph  of  phosphatic  rock.      Note 

Which  are  younger       the  deposition   bands  in  the  lower  right-hand  corner, 
than    the    Ordovi-       (Tenn.  Geological  Survey.) 
cian     and      which 

overlie  them  in  places.      In  contrast   to  the  brown  phosphate   this 
1  Columbia  Folio,  Ex.  by  C.  W.  Hayes  and  E.  O.  Ulrich,  U.  S.  Geological  Survey. 


THE  TENNESSEE  PHOSPHATES 


277 


rock  has  not  been  leached,  for  it  is  for  the  most  part  not  exposed; 
it  is  mined  underground  and  is,  therefore,  more  expensive  to 
obtain  than  the  brown  phosphate  which  is  mined  from  open  pits. 
The  lower  Devonian 

beds    are    mostly    of  ^BB^Bffl^^KSfeflMl^ 

black  or  bluish  shale, 

often    termed    "  blue 

phosphate" ;  the  thick- 
ness varies  from  a  few 

inches  to   4  feet  and 

the      percentage     of 

phosphate  is  also  vari- 
able, but   some   beds 

have    the    very   high 

percentage  of  85  per 

cent   phosphoric  acid 

(P2O5).    In  the  upper 

part  of  the  formation 

are     found      smooth, 

dark-colored    nodular 

phosphates       often 

called   "  phosphate 

lumps."     At  present  this  form  is  not  mined  on  account  of  the  expense 

in  mining,  although  some  of  this  bed  is  very  rich. 

This  phosphate  probably  has  about  the  same  origin  as  the  Silurian 

phosphate  except  that  two  special  factors  are  involved.     The  older 

and  underlying  Silurian  phos- 
phatic  rocks  were  uplifted, 
weathered  and  eroded  and 
their  waste  was  carried  into 
the  sea,  as  shown  in  Fig.  262, 
to  form  Devonian  rocks.  This 
Silurian  land  waste  was  some- 
what sorted  by  the  waves  and 
currents  and  some  of  the  finer 
particles  were  washed  away, 


FIG.  261 — Oolitic  phosphate  rock,  Montana.  The 
texture  is  coarser  than  the  average.  (U.  S.  Geologi- 
cal Survey.) 


FIG.  262. — Diagram  to  illustrate  the  deposi- 
tion of  Silurian  phosphatic  waste  in  De- 
vonian seas. 


leaving  some  phosphatic  ma- 
terials. To  this  was  added  the  shells  and  especially  the  fish  remains 
from  the  Devonian  seas,  for  this  period  witnessed  a  great  expansion 
of  fish  life  and  fish  bones  are  highly  phosphatic. 


278 


MINERAL  FERTILIZERS 


The  Florida  Phosphates 

These  consist  of  four  principal  types:    Rock  phosphate,  soft  phos- 
phate;  land  pebble  phosphate  and  river  pebble  phosphate.     The  rock 

phosphate  consists  of  frag- 
ments of  phosphorite  vary- 
ing in  size  from  pebbles 
to  boulders  all  more  or 
less  embedded  in  sand, 
clay  and  soft  phosphate. 
The  proportion  of  rock 
phosphate  to  the  sur- 
rounding materials  varies 
from  50  per  cent  down, 
but  the  commercial  mines 
usually  yield  from  15  to 
25  per  cent.  There  is 
usually  a  considerable 
thickness  of  overburden, 
mostly  of  sand.  The  thick- 
ness of  the  strata  bearing 
the  rock  phosphate  ranges 
up  to  100  feet,  but  the 

usual  thickness  is  much  below  this.     Most  of  the  phosphorite  boulders 
are  somewhat   spherical   and   many  contain  cavities  which  are  often 
lined    with    minerals,   Fig.   264. 
The  larger  boulders  are  blasted 
into  smaller  pieces  for  the  crush- 
ers and  the  pebbles  are  separated 
by  washing.    The  soft  phosphate, 
when  dry,  is  a  powdery  mass  and,       ^ 
when  wet,  it  forms  a  sticky  mass ; 
it  is  not  mined   or  recovered  at 
present.     There  is  no  sharp  dis- 
tinction between  the  rock  phos-      FIG  264^-Fragment  of  a  boulder  of  rock 
i  T  xl          p,     i         i  phosphate  showing  part  of  a  cavity  lined 

phate  and  the  soft  phosphate.,  with  crystalline    phosphate   minerals. 

The  origin  of  the  rock  and  soft         (U.  S.  Geological  Survey.) 
phosphates  is  as  yet  an  unsettled 

question,  but  it  can  be  safely  stated  that,  like  most  other  phosphorites, 
these  are  of  secondary  origin.     The  phosphatic  materials  were  originally 


FIG.  263. — Map  of  Florida  showing  the  principal 
phosphate  areas  (dotted).  (Matson,  U.  S.  Geo- 
logical Survey.) 


THE  FLORIDA  PHOSPHATES 


279 


scattered  through  the  rocks  and  have  been  concentrated  by  the  action 
of  water  either  mechanically  or  chemically.     A  mechanical  concentra- 
tion is  sometimes  found  where  running  water  has  separated  the  phos- 
phatic  pebbles  and  deposited 
them.     The  chemical  work  of 
ground  water  in  these  deposits 
seems  to  consist  largely  in  the 
alteration  of  the  original  cal- 
cium carbonate  in  the  rocks  to 
lime  phosphate,  a  process  due 
to  the  work  of  ground  water 
containing  phosphoric  acid.  In 
most  cases  this  process  changes     FlG  265._phosphatized  iimestone,  Fla.  The 
the  texture  of  the  original  rock         original  fossiliferous  limestone    has    been 
and    destroys   the   fossils,  but          replaced  by  phosphate.     (U.  S.  Geological 
in   some   instances   this    does         Survey.) 
not   occur;     for    instance    in 

Fig.  265  the  fossiliferous  rock  has  been  thoroughly  phosphatized.  The 
most  valuable  deposits  are  the  boulders  and  pebbles  which  appear  to  be 
concretions,  those  spherical  aggregates  which  are  found  more  or  less  in 


^//*\-'.''':'-S''*'-:'-"iv-:'-':'.::''.*'"''  '••'•••'-•••'•,•'"••  •'^''^••; 


PHOSPHATIC  PEBBLES  IMBEDDED 
IN  A  SANDY  MATRIX 


SANDSTONE, 


FIG.  266.^An  occurrence   of  phosphate  in  Florida.     (Matson,  U.  S.  'Geological 

Survey.) 


all  rocks.  For  some  reason  not  clearly  understood,  a  phosphatic  con- 
cretion is  started  and  its  growth  may  be  continued  by  the  materials 
brought  to  it  in  solution  in  ground  water.  In  other  instances  phosphatic 


280  MINERAL   FERTILIZERS 

boulders  were  formed  by  the  accumulation  of  materials  in  pre-existing 
cavities.  A  somewhat  typical  section  of  phosphate  deposits  is  seen  in 
Fig.  266,  where  the  mixture  of  phosphate  boulders  and  pebbles  and 
sand  is  seen  resting  on  an  uneven  surface  of  bed  rock. 

Land  pebble  phosphate  is  a  mass  of  phosphate  pebbles  intermixed 
with  sands  and  clays.  It  was  formed  by  waves  acting  on  phosphate- 
bearing  formations,  sorting  out  phosphatic  pebbles  and  depositing 
and  concentrating  them.  Furthermore,  since  these  pebbles  have  been 
deposited,  there  has  been  some  enrichment  by  ground  water  which  has 
dissolved  and  redeposited  phosphatic  materials. 

The  river  pebble  phosphate  is  a  stream  deposit.  In  places  this 
deposit  shows  rough  stratification  with  intermingled  layers  of  sand, 
sandy  clay  and  phosphatic  pebbles  which  are  usually  rounded.  These 
deposits  occur  in  the  terraces  and  flood  plains  of  streams.  They,  like 
other  stream  deposits,  are  primarily  due  to  the  weathering  of  phos- 
phatic rock,  the  fragments  being  subsequently  carried  and  deposited 
by  streams.  This  type  was  the  first  mined,  but  it  is  of  low  grade  and 
little  mined  at  present  because  the  stream  separation  of  the  phosphatic 
pebbles  was  very  imperfect. 

The  ultimate  sources  of  these  phosphates  are  not  as  yet  clearly  under- 
stood, and  with  present  knowledge  but  little  more  can  be  stated  than 
that  the  phosphatic  materials  were  derived  from  one  or  all  of  the  follow- 
ing sources:  (1)  Original  phosphatic  materials  in  the  limestone,  (2) 
phosphatic  materials  from  formerly  overlying  limestone  and  (3)  phos- 
phoric acid  contained  in  organic  matter  resting  on  the  surface  of  the 
limestone.  Such  organic  matter,  either  guano  or  other  animal  remains, 
is  thought  by  some  to  have  furnished  the  phosphoric  acid  which  com- 
bined with  the  limestone  materials  to  form  the  lime  phosphate.  This 
theory  is  somewhat  strengthened  by  the  frequent  association  of  bones 
and  fish  teeth  with  the  phosphates.  As  in  so  many  other  geological 
problems  the  original  rock  records  have,  for  the  most  part,  been  destroyed 
and  ultimate  origins  are  very  difficult  to  decipher. 

Other  Areas 

Large  areas  of  phosphate-bearing  rocks  have  been  found  in  Idaho, 
Wyoming  and  Utah.  Their  extent  is  not  yet  known  and  at  present 
they  are  but  little  developed  because  of  limited  market  and  lack  of 
transportation  facilities.  The  phosphate  formation  is  from  60  to  100 
feet  thick  and  consists  mainly  of  beds  of  limestones  and  shales.  The 


POTASH  281 

main  phosphate  bed  ranges  from  a  few  inches  to  10  feet  in  thickness, 
but  the  thinner  portions  are  richest.  The  phosphate  content  in  the 
workable  beds  varies  from  65  to  80  per  cent  of  lime  phosphate,  which  is  a 
high  content.  These  beds  have  been  folded  and  faulted  so  that  some 
are  easily  accessible.  This  is  probably  the  largest  area  of  commercial 
phosphatic  rocks  yet  discovered  and  it  constitutes  a  phosphate  reserve 
of  great  future  importance.  Low-grade  phosphate  rock  has  been  mined 
in  Arkansas. 

-v  ('"'•     ?'.-.'•  **•«-*.;' 

POTASH  -4 

Potash  (K2O)  has  a  wide  use  as  a  fertilizer.  It  is  widely  dessem- 
inated  in  soils,  but  in  many  cases  the  soil  potash  is  in  forms  not  readily 
available  for  crops  and  some  soils  are  decidedly  deficient  in  this  food. 
Potassium  is  widely  distributed  through  the  acid  rocks,  especially 
granites  and  syenites,  and  it  is  an  important  constituent  of  gneisses  and 
schists.  In  these  rocks  it  is  mainly  in  the  form  of  orthoclase  (KAlSiaOs), 
a  potash  aluminum  silicate  which  has  a  possible  potash  content  of  19.3 
per  cent.  Orthoclase  is  probably  the  principal  primary  source  of  nearly 
all  potash,  but  the  mineral  is  so  slowly  soluble  and  so  stable  that  no 
method  has  been  found  to  make  its  potash  commercially  available. 
Orthoclase  has  been  ground  and  applied  to  soils  to  a  small  extent  as  a 
potash  fertilizer,  but  the  results  have  not  been  satisfactory  because  the 
mineral  is  so  slowly  soluble.  Another  much  less  widely  distributed 
potash  mineral  is  leucite  (K2AlSi40i2),  a  potash  aluminum  silicate  with 
a  possible  potash  content  of  21.5  per  cent.  Leucite  occurs  in  large 
amounts  in  fine-grained  lavas  of  the  Leucite  Hills  of  southwestern 
Wyoming,  but  so  far  no  practicable  method  has  been  devised  to  make  its 
potash  available.  Both  orthoclase  and  leucite  constitute  enormous 
potash  reserves  if  practicable  methods  can  be  devised  to  make  their 
potash  available. 

Glauconite,  often  called  "  greensand,"  is  a  widely  distributed  potash 
mineral  in  the  Coastal  Plain  from  New  Jersey  into  Virginia.  It  occurs 
in  small  rounded  grains,  usually  dark  green  in  color,  which  are  often 
intermixed  with  sand  and  bits  of  shells  and  is  found  in  beds  varying 
in  thickness  from  a  few  inches  to  as  much  as  30  feet.  The  composition 
is  somewhat  variable,  but  the  mineral  is  essentially  a  silicate  of  iron 
and  potash,  that  is,  it  is  a  compound  of  iron,  potash  and  silica  with  a 
maximum  of  about  7  per  cent  of  potash.  With  its  nearness  to  markets 
and  transportation  routes,  its  fair  potash  content  and  its  easy  mining 


282 


MINERAL  FERTILIZERS 


this  is  a  promising  potash  source,  but  as  yet  no  process  has  been  found 
for  the  cheap  potash  extraction.  It  is  used  locally  as  a  potash  fertilizer, 
but  the  low  potash  content  will  not  admit  of  long-distance  transporta- 
tion. Glauconite  is  being  formed  on  present  sea  bottoms  but  its  origin 
is  not  well  understood.  No  important  supply  of  potash  has  yet  been 
found  in  North  America;  small  amounts  in  the  United  States  have 
been  extracted  from  potash-bearing  brines  and  from  sea  weeds. 

The  Stassfurt  Region. — The  principal  source  of  potash  for  the 
world  is  from  the  Stassfurt  region  in  Germany,  and  the  occurrence 
is  so  important  and  so  interesting  from  a  geological  point  of  view 
that  it  merits  a  somewhat  detailed  consideration.  The  principal 
potash  minerals  of  this  deposit  are  k^inite  (MgSo4,KCl,3H2O),  car- 
nallite  (KMgCl3,6H20)  and  sylvite  (KC1);  in  all,  about  thirty  different 
minerals  have  been  found  in  these  deposits.  It  will  be  seen  that  these 
potash  minerals  are  for  the  most  part  complex  compounds.  They  are 
easily  soluble  in  water,  a  property  that  makes  them  especially  valuable 
in  fertilizers  since  the  potash  compounds  are  readily  available  for  crops. 

There  are  some  disadvantages 
in  these  fertilizers  in  that  they 
usually  contain  considerable 
salt  (NaCl) ;  in  fact,  the  potash 
deposits  were  discovered  when 
boring  for  rock  salt. 

Looking  at  a  section  of  these 
deposits  in  Fig.  267,  one  notes 
that  they  are  roughly  arranged 
in  layers,  although  it  must  be 
remembered  that  the  layers  are 
not  sharply  separated  from  each 
other  and  most  layers  contain 
other  than  the  principal  min- 
eral; for  instance,  practically 
all  the  zones  contain  varying 
amounts  of  salt.  These  deposits 
illustrate  the  general  principle 
of  chemical  deposition  from 
water,  namely  that,  when  water 
is  evaporated,  the  substances  in 

solution  are  deposited  in  reverse  order  of  solubility,  least  soluble  first, 
most  soluble  last.     One  must  imagine  a  more  or  less  enclosed  body  of 


FIG.  267. — Section   of   the  Stassfurt  salts 
beds.     (After  H.  M.  Caddell.) 


NITRATES  283 

water  in  which  the  evaporation  was  in  general  greater  than  the  inflow. 
As  the  waters  became  more  concentrated,  the  rock  salt  was  first  deposited 
in  enormous  quantities.  Upon  this  was  deposited  an  impure  layer  of 
polyhalite  (2CaSO4,MgSO4,K2S04,2H2O),  a  mixture  of  the  sulphates 
of  magnesia  and  potash  together  with  varying  amounts  of  salt.  Then 
as  the  waters  became  more  concentrated,  the  layers  of  kieserite  and 
carnallite  were  deposited.  The  kainite  was  probably  formed  by  the 
action  of  the  ground  waters  after  the  deposition  was  completed.  Such 
a  more  or  less  definite  succession  of  deposits  implies  a  dry  climate,  a 
conclusion  which  is  supported  by  many  other  lines  of  evidence  and, 
moreover,  the  Permian  period  to  which  these  deposits  belong  was 
characterized  the  world  over  by  an  arid  climate. 

The  stratum  of  salt  clay  indicates  a  break  in  the  deposition  when 
conditions  of  climate  or  topography,  or  perhaps  both,  combined  to  bring 
about  a  deposition  of  clay.  After  this  the  arid  conditions  again  set  in 
and  anhydrite  (CaSCU)  was  deposited  in  the  waters  which  were  becom- 
ing concentrated  by  evaporation  and  in  some  places  rock  salt  was 
deposited  on  the  anhydrite  as  the  waters  became  more  concentrated. 
Finally,  rainfall  increased  and  clays  and  sands  were  washed  in  and  now 
form  the  clays,  shales  and  sandstones.  It  is  still  an  unsettled  question 
as  to  what  were  the  conditions  by  which  the  waters  contributing  these 
deposits  were  surrounded.  Since  it  is  known  that  similar  geological 
conditions  existed  elsewhere  during  the  Permian  and  other  periods,  it 
would  seem  that  other  extensive  potash  deposits  may  be  found,  but  as 
yet  none  of  the  extent  of  the  Stassfurt  deposits  have  been  discovered. 


NITRATES 

Nitrates  are  mined  for  fertilizers  and  for  many  other  purposes. 
The  principal  commercial  nitrate  is  soda  nitre  (NaNOs)  and  the  main 
supply  comes  from  northern  Chili,  where  the  nitre  occurs  in  beds  up 
to  6  feet  in  thickness.  The  crude  material,  known  locally  as  caliche,  is 
always  impure,  the  commercial  beds  running  from  25  to  50  per  cent  of 
nitre.  The  origin  of  the  nitrates  is  an  unsettled  question.  The  dry 
climate  in  which  these  deposits  occur  is  an  important  factor  in  their 
preservation  since  the  nitre  is  readily  soluble  and  much  would  be  dis- 
solved and  carried  away  in  a  humid  climate. 


284  MINERAL  FERTILIZERS 


GYPSUM  AND  LIMESTONE 

Gypsum  (CaSO^EkO),  often  known  as  "  land  plaster,"  is  applied 
to  a  considerable  extent  to  alkali  soils  in  some  regions  of  small  rainfall. 
The  gypsum  combines  with  the  alkaline  compounds  making  them  less 
harmful  to  crops.  It  is  also  used  in  the  manufacture  of  plaster  of 
Paris,  stucco  and  some  cements  and  plasters.  Land  plaster  is  usually 
impure  gypsum  which  contains  more  or  less  sand  and  clay. 

Gypsum  occurs  in  commercial  quantities  only  in  sedimentary  rocks, 
beds  of  gypsum  often  alternating  with  beds  of  limestone.  Most  salt 
deposits  are  more  or  less  associated  with  gypsum,  but  not  all  gypsum 
deposits  are  associated  with  salt.  The  extensive  beds  of  gypsum,  like 
those  of  salt,  are  thought  to  be  due  to  precipitation  from  evaporating 
sea  water.  Gypsum  begins  to  precipitate  when  about  one-third  of 
enclosed  sea  water  has  been  evaporated  and  the  process  has  usually 
been  interrupted  before  the  salt  and  other  easily  soluble  minerals  have 
been  deposited,  hence  gypsum  is  often  not  associated  with  salt.  It  is  a 
widely  distributed  mineral  occurring  in  many  parts  of  the  world  and 
in  many  formations  from  among  the  oldest  to  the  youngest. 

Limestones. — The  origin  and  properties  of  limestone  have  been 
discussed  elsewhere.  Large  amounts  of  limestone  are  crushed  and 
ground  fine  for  application  to  soils,  mainly  to  correct  soil  acidity. 
The  use  of  limestone  for  this  purpose  is  rapidly  increasing. 

REFERENCES 

A.  W.  GRABAU,  The  Principles  of  Stratigraphy,  Seiler,  1913:    Greensand,  pages 

670-673. 

E.  W.  HILGARD,  Soils,  Macmillan,  1911,  Chapter  5.     (Fertilizers.) 
H.  RIES,  Economic  Geology,  Wiley,  1916,  Fertilizers,  Chapter  8. 

Phosphates 

C.  W.  HAYES,  The  Tennessee  Phosphates,  17th  Ann.  Rept.,  Part  2,  U.  S.  Geological 

Survey,  1896,  pages  513-550. 

G.  C.  MATSON,  The  Florida  Phosphates,  Bull.  604,  U.  S.  Geological  Survey,  1915. 
E.  H.  SELLARDS,  Various  Reports  of  the  Florida  Geological  Survey. 
"Resources  of  Tennessee,"  issued  by  the  Tennessee  Geological  Survey. 


CHAPTER   XIV 
SOIL  REGIONS   OF  THE   UNITED   STATES1 


Introductory.  —  It  is  not  possible  to  map  divisions  within  which  the 
soils  are  absolutely  distinctive,  but  for  many  purposes  the  soils  of  the 
United  States  may  be  described  under  thirteen  divisions.  Soils  are 
influenced  for  the  most  part  by  three  factors;  (1)  the  soil  materials; 


*('    lo      >      / 1 —         ! 


\Y,.£L_  / /2/    r~  "~tx^\, 6 1  i^ 


FIG.  268.— Soil  map  of  the  United  States.     (U.  S.  Bureau  of  Soils.) 

(2)  the  processes  by  which  the  soils  have  originated;  (3)  the  processes 
in  operation  since  the  soils  were  formed.  It  is  obvious  that  all  these 
factors  may  be  in  operation  in  different  divisions  but  their  combinations 
may  be  so  different  that  the  resulting  soils  may  be  somewhat  dis- 
tinctive. Furthermore,  climate  must  be  recognized  also  as  an  impor- 
tant factor,  both  because  of  its  crop  interest  and  because  of  its  important 
effects  on  soils;  soils,  for  example,  from  the  same  kinds  of  rocks  in 
1  Abstracted  and  slightly  abbreviated  from  Bulletin  No.  96,  U.  S.  Bureau  of  Soils. 

285 


286  SOIL  REGIONS  OF  THE  UNITED  STATES 

North  Carolina  arid  in  Pennsylvania  differ  because  of  climatic  differences. 
Each  soil  division  has  a  somewhat  distinctive  topography  and  its  soils 
have  resulted  from  closely  associated  and  somewhat  distinctive  factors. 
The  map,  Fig.  268,  shows  the  principal  soil  divisions  as  determined  by 
the  U.  S.  Bureau  of  Soils. 

The  Coastal  Plain  (page  265)  is  one  of  the  most  important  soil 
regions  of  North  America.  It  extends  in  crescent-like  shape  from  New 
Jersey  through  Texas  with  characteristic  soils  but  with  climate  varying 
cool  to  semi-tropical  and  from  humid  to  dry.  The  general  structure  is 
shown  in  Figs.  250  and  251,  where  it  is  seen  that  the  underlying  forma- 
tions dip  gently  towards  the  Atlantic  and  Gulf.  The  rocks  are  mostly 
unconsolidated  sands  and  clays,  although  there  are  large  areas  underlain 
by  soft  limestones  and  marly  clays  and  still  other  areas  are  underlain 
by  sandstones.  These  materials  have  been  washed  down  from  the 
higher  lands  to  the  north  and  west  and  were  deposited  in  the  ocean. 
The  ocean  bed  was  subjected  in  complicated  emergences  and  sub- 
mergences, but  finally  a  part  of  the  ocean  bed  was  elevated  and  now 
forms  the  Coastal  Plain.  Still  later  much  of  the  Coastal  Plain  was 
overspread  by  a  sandy  formation,  the  Lafayette  Formation,  which 
yields  many  of  the  loams,  sandy  loams  and  sands  of  this  division.  As 
a  whole  the  soils  are  sandy,  but  varying  conditions  of  drainage  give  a 
considerable  variety  of  soils.  A  belt  in  Alabama,  Mississippi  and  Texas 
is  underlain  by  friable  limestone  which  yields  the  productive  soils  of  the 
Black  Belt  (see  Fig.  251).  As  a  whole  the  surface  is  level  to  rolling, 
although  markedly  hilly  regions  are  not  lacking  in  the  older  and  upper 
parts;  a  fairly  typical  view  is  shown  in  Fig.  253.  The  climate  ranges 
from  the  cool,  humid  climate  of  New  Jersey  to  the  warm  dry  climate  of 
Texas,  but  the  larger  areas  are  located  in  the  South  with  a  warm  climate. 

The  Piedmont  Plateau  lies  west  and  north  of  the  Coastal  Plain 
as  will  be  apparent  from  the  map.  As  a  whole  the  topography  is 
strongly  rolling  rather  than  hilly  and,  even  in  areas  of  high  hills,  the 
slopes  are  often  so  moderate  that  tillage  is  possible  from  hill  base  to 
summit.  Between  the  Coastal  Plain  and  the  Piedmont  Plateau  is  the 
so-called  "  fall  line  "  or  more  properly,  the  fall  zone  in  which  streams 
descend  from  the  Piedmont  Plateau  by  steeper  slopes  or  by  falls.  The 
region,  as  a  whole,  is  in  the  mature  stage  of  erosion  and  is  well  drained 
by  numerous  streams.  For  the  most  part,  the  soils  are  residual  and 
correspond  rather  closely  to  the  corresponding  underlying  rocks  which 
are  mainly  igneous  and  metamorphic.  The  soils  are  in  consequence 
rather  heavy,  consisting  largely  of  loams,  clay  loams  and  clays.  The 


THE  APPALACHIAN  MOUNTAIN  AND  PLATEAU  REGION    287 

prevailing  soil  color  in  the  southern  parts  is  reddish  and  this  division 
is  often  known  as  the  "  Red  Land  Country."  The  total  area  of  sandy 
soils  is  small. 

The  Appalachian  Mountain  and  Plateau  Region  is  more  diversified 
than  either  of  the  two  preceding  divisions.  The  main  area  includes 
three  sub-divisions  as  follows:  The  Blue  Ridge  Region  on  the  east  and 
southeast,  the  Cumberland-Allegheny  Plateau  on  the  west  and,  between 
these  the  Appalachian  Valley  and  Ridge  Belt.  These  are  shown  in 
Fig.  269.  Besides  the  main  area,  there  are  two  subordinate  areas, 
one  a  large  area  in  the  Ouachita  and  Boston  Mountain  regions  of 
Arkansas  and  Oklahoma  and  the  other,  a  small  area  in  western  Ken- 
tucky. A  small  part  of  this  region  has  been  glaciated  and  its  soils  will 
be  considered  later. 


CUMBERLAND  PLATEAU 


APPALACHIAN  VALLEY 
AND  RIDGE  BELT 


BLUE  RIDGE  MTS. 


FIG.  269. — Diagram  to  illustrate  the  topography  and  structure  of  the  Cumberland 
Plateau,  Appalachian  Valley  and  Ridge  Belt  and  the  Blue  Ridge  Mountains. 


The  Blue  Ridge  Belt  resembles  the  Piedmont  Plateau  in  its  rocks 
and  general  geology,  the  main  difference  being  one  of  topography  and 
altitude.  The  Blue  Ridge  proper  is  hilly  to  mountainous,  but  often 
there  is  such  a  gradation  between  the  two  divisions  that  no  definite 
dividing  line  can  be  drawn  and,  in  fact,  it  is  only  from  Maryland  south- 
ward that  the  Blue  Ridge  becomes  a  markedly  distinct  feature.  In 
North  Carolina,  the  highest  mountains  east  of  the  Mississippi  are  found 
in  the  Blue  Ridge.  The  slopes  in  general  are  steep  and  the  soils  thin 
and  stony  although  here  and  there  are  rolling  plateaus  with  soils  much 
like  those  of  the  Piedmont  Plateau.  The  main  agricultural  areas 
are  located  on  the  valley  soils.  Much  of  the  area  is  in  timber  and  for- 
estry should  probably  be  the  main  industry  for,  when  the  timber  is  cut, 
the  soils  are  very  easily  eroded. 


288  SOIL  REGIONS  OF  THE  UNITED  STATES 

Lying  west  and  generally  distinct  from  the  Blue  Ridge  is  the  Appa- 
lachian Ridge  Belt,  which,  as  the  name  implies,  is  characterized  by  long, 
narrow  ridges  separated  by  valleys.  The  rocks  are  for  the  most  part 
much  vaulted  and  so  intricately  folded  that  the  same  rock  repeatedly 
appears  at  the  surface  in  narrow  outcrops,  Fig.  49.  The  more  resistant 
rocks  like  sandstones  and  some  limestones  have  been  less  slowly  eroded 
and  stand  as  ridges  while  the  weaker  shales  and  limestones  are  eroded 
to  valleys,  Figs.  56,  57  and  58.  Naturally  the  main  arable  soils  are  found 
in  the  shale  valleys,  the  soils  of  which  are  usually  very  productive. 
The  ridges  are  often  stony  and  non-arable  and  the  soils  are  thin  and 
frequently  stony;  they  are  mainly  used  for  pasture,  fruit  trees  or 
forestry. 

The  Cumberland- Allegheny  Plateau  is  seldom  as  level  as  the  name 
might  imply;  the  region  was  once  a  plateau,  but  it  has  been  so  maturely 
eroded  that  only  comparatively  small  areas  of  level  land  remain.  Much 
of  the  region  is  markedly  hilly,  so  rough,  indeed,  that  the  term  mountains 
is  commonly  applied  to  much  of  the  region.  The  soil-forming  rocks  are 
largely  shales  and  sandstones  and  typically  the  soils  are  sandy.  The 
prevalence  of  steep  slopes  leads  to  soil  creep,  colluvial  soils  commonly 
extend  around  the  lower  slopes  of  hills  and  soil  erosion  is  an  important 
problem.  Stony  soils  and  rocky  areas  are  common. 

The  Limestone  Valleys  and  Uplands  occur  in  the  western  parts  of 
Tennessee  and  Kentucky  and  also  in  the  southern  half  of  Missouri 
and  the  northern  part  of  Arkansas.  The  Kentucky-Tennessee  region 
has  two  main  features,  the  central  limestone  basin  of  Tennessee  and 
the  similar  basin  in  Kentucky  known  as  the  "  Bluegrass  Country." 
Both  are  limestone  basins  more  or  less  enclosed  by  higher  country  known 
as  the  "  Highland  Rim."  A  section  across  one  of  these  basins  is  shown  in 
Fig.  59.  The  limestone  basins  have  a  rolling  to  hilly  topography, 
they  are  well  drained  and,  in  general,  the  soils  are  deep  and  heavy. 
The  topography  of  the  Highland  Rim  is  much  rougher,  varying  from 
rolling  to  much  eroded  surfaces.  The  underlying  rocks  are  generally 
somewhat  cherty  limestones  and  the  soils  are  often  stony  both  because 
of  their  marked  slopes  and  because  of  their  chert  fragments.  The  best 
lands  are  found  in  valleys  and  "  coves." 

The  Ozark  uplands  in  Missouri  and  Arkansas  are  underlain  by  a 
huge  dome-shaped  uplift  formerly  a  plateau,  but  now  much  eroded  so 
that  the  old  fairly  level  plateau  surface  is  preserved  only  in  patches, 
while  the  remaining  surfaces  are  more  or  less  hilly  and  rugged.  The 
underlying  rocks  are  largely  limestones,  many  of  them  cherty,  and  the 


THE  GLACIAL  AND  LOESSIAL  SOIL  REGIONS  289 

soils  are  prevailingly  silty  loams,  loams  and  clay  loams  with  stony 
soils  from  the  cherty  limestones. 

The  glacial  and  loessial  soil  regions  constitutes  a  large  and  impor- 
tant area  which  shows  considerable  soil  and  topographic  diversity. 
This  diversity  is  due  to  several  factors:  (1)  The  close  association  of 
glaciation  with  loessial  soils  has  been  noted  (page  234).  (2)  It  will  be 
remembered  that  there  is  often  a  close  association  of  glacial  materials 
with  the  underlying  rocks  so  that  one  would  expect  considerable  soil 
diversity.  (3)  The  glaciated  region  extends  in  a  general  east-west 
direction  and  so  cuts  across  several  other  divisions  the  northern  por- 
tions of  which  have  been  glaciated.  (4)  Each  division  in  general  pre- 
serves its  main  topographic  features,  for  the  glaciers  did  not  greatly 
modify  these  features,  but  the  minor  features  and  especially  the  drainage 
were  often  much  changed.  Probably  any  soils  rather  than  the  topog- 
raphy suffered  the  greatest  changes  due  to  glaciation.  It  is  convenient 
to  consider  these  glacial  and  loessial  divisions  under  the  following  heads : 

The  Interior  Lowland  Area  includes  the  glaciated  areas  west  of  the 
Allegheny  Plateau.  This  can  be  separated,  both  as  to  soils  and  to 
topography,  into  two  divisions,  (1)  the  southern  and  (2)  the  northern 
areas.  The  southern  area  is  characteristically  level  or  rolling  and  most 
of  the  soils  are  loessial;  these  soils  show  comparatively  little  variation 
from  place  to  place.  It  is  an  important  wheat  and  corn  country.  The 
northern  area  is  predominantly  glacial  although  there  is  no  clear  divid- 
ing line  between  the  areas.  It  has  a  rougher  topography,  but  seldom 
too  rough  for  cultivation.  The  soils  are  much  more  varied,  not  only 
because  the  glaciers  carried  and  deposited  varied  materials,  but  also 
because  of  the  action  of  water  derived  from  melting  ice. 

The  Western  Glaciated  Area  is  much  more  diverse  in  topography  and 
it  includes  practically  no  loessial  soils.  The  northern  part  of  the  Cum- 
berland-Allegheny Plateau,  mainly  in  New  York  and  northern  Penn- 
sylvania, has  been  glaciated.  In  the  main,  the  surface  is  hilly  and 
mostly  composed  of  rather  steep  slopes,  but  a  large  percentage  of  the 
land  is  arable.  The  soils  are  somewhat  thin  and  show  a  close  relation 
to  the  underlying  sandstones  and  shales.  Lying  to  the  eastward  is  a 
small  portion  of  the  Appalachian  Ridge  Belt.  The  ridges  have  gener- 
ally been  swept  almost  bare  of  their  soils,  but  the  valley  soils  are  pro- 
ductive. In  New  England  glaciation  was  especially  vigorous,  much  of 
the  preglacial  soil  was  removed  and,  as  a  rule,  only  thin  soils  were 
deposited  at  the  glacial  retreat.  The  granites  and  similar  rocks  have 
resisted  glacial  erosion  so  that  the  soils  are  characteristically  stony. 


290  SOIL  REGIONS  OF  THE  UNITED  STATES 

Northern  New  England  is  hilly  and  mountainous  and  much  is  too  rough 
for  cultivation.  The  Connecticut  Valley,  which  extends  through 
southern  New  England,  is  floored  by  fairly  productive  soils,  mostly 
terrace  soils. 

The  Great  Plains  Region  is  a  large  area  including  a  considerable 
variety  of  soils,  but  in  the  main  having  a  level  to  undulating  surface. 
On  the  east,  the  region  lies  at  elevations  of  from  1000  to  1200  feet  from 
which  it  rises  very  gently  to  the  westward  to  elevations  of  from  4000 
to  6000  feet.  Perhaps  the  most  important  soil  and  agricultural  feature 
is  the  decrease  in  rainfall  from  east  to  west;  in  the  eastern  parts,  the 
rainfall  is  ordinarily  ample  while  the  western  portions  are  semi-arid. 
It  has  been  noted  (page  174)  that  the  western  part  of  this  region  is 
more  or  less  covered  with  materials  washed  eastward  from  the  Rocky 
Mountain  region.  To  the  eastward  of  the  areas  covered  by  these  mate- 
rials, the  soils  are  mainly  residual  from  sedimentary  rocks. 

The  Rocky  Mountain  and  Plateau  Region  lies  in  a  region  of  scant 
•rainfall  and  sharp  slopes.  The  soils  are  subject  to  washing  and  to  soil 
creep.  As  a  whole  the  region  is  best  adapted  to  grazing.  The  North- 
western Intermountain  Region  typically  contains  two  classes  of  soils: 
those  derived  from  comparatively  recent  lava  flows,  which  have  been 
noted  on  preceding  pages  (page  36),  and  the  soils  in  depressions  formerly 
occupied  by  lakes.  All  the  soils  have  been  greatly  modified  by  wind 
action,  which  is  especially  effective  because  of  the  dryness  of  the  climate. 
Many  of  these  soils  are  very  productive  where  irrigation  water  is  avail- 
able since  their  mineral  plant  food  has  not  been  leached. 

The  Great  Basin  for  the  most  part  is  characterized  by  interior 
drainage,  that  is,  the  escape  of  water  from  the  region  is  mainly  by  evap- 
oration. It  is  a  region  of  scant  precipitation  except  in  the  mountains 
which  fringe  the  basin  and  the  cultivated  lands  are  therefore  mainly 
along  the  bases  of  mountains  and  along  the  short  rivers  where  irrigation 
water  is  readily  procured.  Numerous  isolated  mountain  ranges  and 
ranges  running  in  a  north-south  direction  rise  abruptly  from  the  loose 
materials  of  the  basin  floor.  Here,  it  will  be  remembered,  were  formerly 
vast  fresh-water  lakes  which  have  disappeared  or  are  represented  by 
shallow  water,  brackish  lakes,  Fig.  233.  The  soils  are  varied,  including 
residual  soils  from  various  rocks,  lake  soils,  wind-blown  soils  and  delta 
and  alluvial  fan  soils  which  extend  outward  from  the  mountain  bases. 
The  limiting  agricultural  factor  is  the  small  amount  of  water  available 
for  irrigation  and  most  of  the  agricultural  lands  are  suitable  only  for 
grazing.  These  agricultural  conditions  are  mostly  duplicated  in  the 


THE  PACIFIC  REGION  291 

Arid  Southwest  Region,  except  that  the  latter  is  as  a  whole  higher. 
Alluvial  soils  are  important  locally. 

The  Pacific  Region  roughly  consists  of  two  mountainous  areas  sep- 
arated by  lowlands.  Along  the  Pacific  coast  the  Coast  Ranges  extend 
in  a  general  north-south  direction.  These  ranges  are  pierced  by  two 
openings,  one  at  San  Francisco  which  leads  to  the  Valley  of  California 
and  the  other  farther  north  which  leads  to  the  Puget  Sound  Lowland. 
East  of  these  lowlands  are  the  Sierra  Nevada  and  Cascade  Mountains. 
As  a  whole,  the  mountains  are  non-agricultural  regions  and  the  main 
agricultural  activities  are  confined  to  the  two  lowlands. 

There  is  an  interrupted  valley  extending  from  Puget  Sound  south- 
ward to  the  Gulf  of  California,  a  valley  interrupted  by  the  Klamath 
Mountains  of  northern  California  and  southern  Oregon.  Puget  Sound 
at  the  north  and  the  Gulf  of  California  at  the  south  appear  to  be  drowned 
portions  of  this  valley.  The  Valley  of  California,  Fig.  153,  which  lies 
south  of  the  Klamath  Mountains,  is  a  somewhat  unique  region  enclosed 
by  high  mountains  with  a  narrow  opening  at  San  Francisco  through 
which  the  waters  of  the  Sacramento  and  San  Joaquin  escape  to  the 
Pacific.  The  valley  is  about  400  miles  long,  50  miles  wide  and  contains 
something  like  20,000  square  miles.  Typically,  the  soils  and  topography 
are  somewhat  simple  and  are  arranged  in  longitudinal  belts.  The 
streams  descending  from  the  mountains  on  the  east  and  v/est  are  rapid, 
carrying  both  fine  and  coarse  debris  and  have  built  alluvial  fans  out- 
ward from  the  mountain  bases,  thus  forming  plains  which  slope  gently 
from  the  mountains,  Fig.  152.  Between  these  sloping  plains  are  the 
flood  plains  of  the  San  Joaquin  and  Sacramento  Rivers  and  in  addition 
there  are  scattered  areas  of  residual  soils.  This  long  valley  extends 
from  southern  California,  with  a  scant  rainfall  to  the  northern  part  of 
the  State,  which  has  a  fairly  abundant  rainfall. 

The  northern  extension  of  this  valley  north  of  the  Klamath  Moun- 
tains may  be  considered  under  two  divisions,  the  Willamette  River 
Valley  to  the  south  and  the  Puget  Sound  region  to  the  north.  The 
Willamette  Valley  soils  consist  of  alluvial  belts  along  the  streams  and 
residual  soils  on  the  lower  foothills.  Much  of  the  Puget  Sound  Valley 
has  been  glaciated  and  glacial  materials  form  most  of  the  soils.  Both 
valleys  have  adequate  rainfall. 


292  SOIL  REGIONS  OF  THE  UNITED  STATES 


REFERENCES 

ISAIAH  BOWMAN,  Forest  Physiography,  Wiley,  New  York,  1911,  Part  2.  The  most 
complete  account  of  the  physiographic  divisions  of  the  United  States  yet  pub- 
lished in  one  volume. 

G.  N.  COFFEY,  A  Study  of  the  Soils  of  the  United  States,  Bull.  85,  U.  S.  Bureau  of 
Soils,  1913. 

W.  J.  McGEE,  The  Lafayette  Formation,  12th  Ann.  Kept.,  U.  S.  Geological  Survey, 
1891. 

U.  S  Bureau  of  Soils,  Bulletin  96,  Soils  of  the  United  States,  1913.  Contains  descrip- 
tions of  the  soils  of  the  United  States  so  far  as  they  have  been  surveyed.  By  far 
the  most  complete  description  yet  published. 


CHAPTER  XV 
HISTORICAL   GEOLOGY 

Historical  geology  deals  mainly  with  (1)  the  growth  and  develop- 
ment of  continents  and  (2)  the  evolution  of  the  earth's  life  as  shown  by 
fossils.  Geological  history  deals  with  enormous  periods  of  time  and, 
as  in  human  history,  many  of  the  records,  especially  the  early  records, 
have  been  obliterated.  It  is  convenient  to  separate  geological  history 
into  more  or  less  clearly  marked  divisions  of  which  the  following  are 
in  use: 

Era.  Period. 

(  Quaternary 

Cenozoic \  ^    , 

I  Tertiary 

Cretaceous 

, .  Comanchean 

Mesozoic T 

Jurassic 

Triassic 

f  Permian 
Pennsylvanian 
Mississippian 


Paleozoic I  Devonian 

Silurian 

Ordovician 

Cambrian 

Pre-Cambrian .  .  Pre-Cambrian 


THE  PRE-CAMBRIAN   ERA 

These  are  the  earth's  oldest  rocks.  They  are  practically  without 
fossils  or  other  evidences  of  life  and  are  predominantly  igneous  and 
metamorphic,  although  there  are  large  areas  of  sandstones  and  con- 
glomerates. These  rocks  are  most  extensively  exposed  in  eastern 
Canada,  but  there  are  considerable  areas  in  the  Adirondacks,  much  of 
New  England,  the  Piedmont  Plateau  and  in  many  parts  of  the  Rocky 

293 


294 


HISTORICAL  GEOLOGY 


Mountains.  They  underlie  all  other  rocks.  Probably  the  most 
important  soils  derived  from  these  rocks  are  those  of  the  Piedmont 
Plateau  (page  286).  The  Canadian  area  is  the  largest,  but  climate  and 
severe  glaciation  have  made  many  of  these  soils  non-agricultural.  The 
copper  and  iron  of  the  Lake  Superior  region  occur  in  these  rocks. 


THE  PALEOZOIC   ERA 

Cambrian  Period. — Life  even  in  this  early  period  was  well  developed 
for  such  well-developed  animals  as  the  complex  trilobite,  Fig.   270, 
.  .    _ ,.n         somewhat  related  to  our  modern  crab,  is 

found  in  these  rocks.  Indeed,  with  the 
exception  of  the  vertebrates  (back- 
boned animals),  all  the  great  divisions 
of  modern  animals  are  represented  in 
the  Cambrian,  but  usually  in  simple  and 
primitive  forms.  Cambrian  rocks,  for 
the  most  part,  are  buried  beneath 
younger  rocks  which  have  been  depos- 
ited above  them.  These  rocks  are  ex- 
posed around  the  edges  of  the  pre- 
Cambrian  rocks  in  New  York,  Canada 
and  Wisconsin  and  in  the  Appalachians, 
where  folding  has  brought  the  deep 
lying  rocks  to  the  surface.  The  rocks 
of  this  period  change  to  those  of  the 
overlying  Ordovician  period  with  no 
abrupt  change  in  fossils.  Much  of 
North  America  was  submerged  during 
this  period  and  in  consequence  the  Ordo- 
vician rocks  are  widespread,  although 
this  does  not  mean  that  they  have  a 

wide  surface  exposure.     Most  of  the  Tennessee  phosphates  occur  in 
these  rocks. 

The  Silurian  Period  followed  without  any  notable  change.  Lime- 
stones were  extensively  formed,  although  shales  and  sandstones  are 
common.  A  somewhat  arid  climate  prevailed,  at  least  locally,  for 
extensive  salt  deposits  were  accumulated  in  the  central  New  York 
region.  Another  important  formation  of  this  period  is  the  Clinton, 
which  extends  from  New  York  to  Alabama  and  locally  yields  much 


FIG.  270.— A  cambrian  trilobite, 
Bathyuriscus  rotundatus  Ro- 
minger.  (Walcott,  Smithsonian 
Institution.) 


THE  PALEOZOIC  ERA  295 

iron  ore.  Fragments  of  fishes  have  been  found  showing  that  they  lived 
during  this  period.  Silurian  and  Ordovician  rocks  in  southeastern 
Canada  and  northern  New  York  furnished  materials  to  the  glaciers 
so  that  these  formations  have  had  considerable  influence  in  the  soils  of 
these  regions  and  even  beyond  them.  Long  belts  of  residual  soils 
from  these  rocks  are  found  in  the  Appalachian  valleys. 

The  Devonian  Period  differs  but  little  from  the  preceding  Silurian. 
The  Devonian  seas  covering  portions  of  North  America  varied  in  size. 
The  rocks  in  New  York,  Pennsylvania  and  West  Virginia  contain 
important  oil  and  gas  formations.  This  period  is  especially  notable 
for  its  number  and  variety  of  fishes,  all  primitive  types  as  compared 
with  modern  fishes.  These  rocks  yield  soils  in  southern  New  York,  in 
Ohio  and  Indiana  and  in  long,  narrow  areas  in  the  Appalachian  Ridges. 

The  Mississippian,  Pennsylvanian  and  Permian  were  formerly 
included  under  the  name  Carboniferous  because,  during  this  time,  the 
great  coal  beds  were  accumulated.  The  early  Mississippian  period 
was  a  time  of  widely  extended  seas  in  which  west  of  Ohio  there  were 
accumulated  widespread  beds  of  limestone,  some  of  which  now  yield 
large  soil  areas  in  the  upper  Mississippi  Basin.  Toward  the  close  of 
this  period  most  of  the  eastern  portions  of  North  America  were  above 
the  seas. 

During  most  of  the  Pennsylvanian  period,  the  land  in  eastern  North 
America  was  low  and  marshy  and  the  conditions  were  favorable  for 
coal  formation.  These  favorable  conditions  were  (1)  an  abundance- 
of  vegetation  and  (2)  favorable  conditions  for  accumulation.  The 
vegetation  was  mostly  of  tree-like  ferns  which  but  little  resembled  the 
vegetation  of  to-day.  There  are  accumulations  of  peat  in  the  swamps 
and  marshes  to-day  and  these  afford  an  explanation  of  the  formation  of 
coal.  When  leaves,  trunks,  twigs,  etc.,  fall  to  dry  ground,  they  decay, 
most  of  their  vegetable  matter  turns  to  water  and  carbon  dioxide,  both 
of  which  escape  into  the  air  and  only  the  mineral  matter  or  "  ash  " 
remains.  When,  however,  the  vegetable  matter  falls  into  water,  the 
decay  is  incomplete  and  the  vegetation  changes  into  peat,  as  has  been 
noted  before  (page  256).  If  the  peat  beds  sink  at  about  the  same  rate 
as  the  peat  is  accumulated,  the  accumulations  may  become  very  thick 
and  very  slowly  the  pressure  of  overlying  materials  compresses  the  peat 
into  lignite  and  finally  into  bituminous  coal.  It  is  obvious  that  many 
feet  of  peat  were  required  to  make  one  foot  of  coal  and  the  formation  of 
coal  beds  required  tens  of  thousands  of  years.  Coal  beds  are  prac- 
tically always  separated  by  beds  of  clay,  shales,  sandstones  and  even 


296  HISTORICAL  GEOLOGY 

limestones,  showing  that  the  coal  swamps  were  subjected  to  many  oscil- 
lations. 

The  Permian  rocks  succeeded  the  Pennsylvanian  without  any 
break,  and  the  main  differences  were  in  the  life  as  shown  by  the  fossils. 
This  period  the  world  over  was  one  of  large  land  areas.  Three  interest- 
ing and  important  events  occurred  in  this  period.  There  was  remark- 
ably widespread  glaciation  even  in  Africa  and  India  and  the  areas 
subjected  to  glaciation  were  much  more  widespread  than  in  the  later 
glacial  period,  which  has  been  considered  at  some  length.  Again,  in 
widely  separated  areas  there  are  thick  beds  of  salt  and  gypsum,  deposits 
that  indicate  an  arid  climate.  Finally,  towards  the  close  of  the  period, 
the  belt  now  occupied  by  the  Appalachian  Ridges  was  folded  and 
faulted,  thus  making  the  beginning  of  this  important  topographical 
region.  It  should  be  remembered,  however,  that  the  present  ridges 
and  valleys  of  this  region  are  not  those  which  were  formed  during  Per- 
mian time,  for  the  rocks  have  repeatedly  been  elevated  and  then  worn 
down;  the  present  rock  structure  here  is  the  result  of  the  Permian  fold- 
ing. As  might  be  expected,  the  period  is  characterized  by  an  expan- 
sion of  land  life,  especially  plants.  The  Amphibians  which  are  to-day 
represented  by  frogs  and  salamanders  attained  large  size  utterly  unlike 
the  present  forms.  The  Pennsylvanian  and  Permian  rocks  are  largely 
sandstones  and  shales;  they  appear  at  the  surface  in  northwestern 
Texas,  Oklahoma,  Missouri,  Illinois  and  also  in  narrow  belts  from 
Pennsylvania  to  Alabama. 

THE  MESOZOIC  ERA 

The  Mesozoic  is  an  era  in  which  the  life,  both  plants  and  animals, 
increasingly  becomes  more  modern.  The  era  is  often  called  the  age  of 
reptiles  because  of  the  development  and  predominance  of  this  form  of 
life.  It  is  divided  as  follows: 

Cretaceous  (Upper  Cretaceous). 

Comanchean  (Lower  Cretaceous). 

Jurassic. 

Triassic. 

The  Triassic  and  Jurassic  rocks  are  unimportant  so  far  as  covered 
areas  in  North  America  are  concerned.  There  are  long,  narrow  areas, 
mostly  red  sandstones  and  shales,  extending  from  New  Jersey  into 
North  Carolina  and  there  is  an  important  area  in  the  Connecticut  Val- 
ley. There  are  also  considerable  areas  in  Utah  and  Arizona.  The 


THE  CENOZOIC  ERA  297 

distinctive  animals  of  these  periods  are  reptiles  which  attained  great 
size  and  diversity.  There  were  also  very  primitive  mammals.  A 
most  interesting  creature  of  these  periods  was  the  pterosaur,  or  "flying 
dragon,"  as  it  is  sometimes  called,  which  had  hollow  bones  and  also 
wings  and  was  the  predecessor  of  our  birds.  There  was  extensive  folding 
of  the  rocks  in  the  Pacific  region. 

The  Comanchean  Cretaceous  period  began  with  large  land  areas  in 
North  America  but,  during  these  periods,  the  continent  sank  in  places 
so  that  there  was  a  vast  marine  invasion  from  the  Gulf  of  Mexico  to 
the  Arctic,  the  last  great  submergence  of  the  continent.  It  was  in 
this  inland  sea  that  the  long  belt  of  Cretaceous  rocks  from  Texas  nearly 
to  Alaska  was  deposited.  These  rocks  underlie  millions  of  acres  in  the 
Middle  West  and  yield  many  of  the  soils  of  the  Great  Plains.  Gradually 
this  interior  sea  became  shallow  and  beds  of  lignite  were  accumulated 
over  large  areas.  Finally,  toward  the  end  of  the  Cretaceous,  the  inte- 
rior sea  withdrew  and,  with  some  minor  exceptions,  the  North  American 
continent  assumed  its  present  shape. 

THE  CENOZOIC    ERA 

The  Cenozoic  era  is  divided  into  a  longer  early  Tertiary  period  and 
a  later  shorter  Quaternary  period.  The  era  is  sometimes  called  the  age 
of  mammals  since  these  animals  became  dominant  during  this  time. 
The  life  forms  more  and  more  approach  and  finally  merge  into  those  of 
the  present. 

The  Tertiary  period  was  characterized  by  a  large  land  area  in  North 
America,  an  area  which  was  subjected  to  long  erosion — two  important 
and  far-reaching  events  occurred  during  this  period.  The  forma- 
tions in  the  Rocky  Mountain  region  were  much  folded  and  these 
mountains  were  initiated.  Then,  especially  in  the  West  and  North- 
west, there  was  great  volcanic  activity;  the  Columbia  River  lavas, 
Fig.  28,  were  in  part  outpoured  during  this  period.  The  sediments  of 
much  of  the  Coastal  Plain  (page  265)  were  deposited  and  these  sediments 
now  underlie  much  of  this  division.  Extensive  Tertiary  deposits  were 
laid  down  on  the  land  and  in  lakes,  for,  it  will  be  remembered,  much 
of  the  continent  was  above  water  during  this  period.  The  phosphate 
deposits  of  Florida  are  in  part  found  in  Tertiary  rocks.  It  was  dis- 
tinctively a  period  of  mammals  of  which  the  early  forms  were  often  large 
and  much  unlike  our  present  animals,  but,  as  the  period  progressed,  the 
mammals  became  more  modern  in  appearance.  Of  especial  interest 


298  HISTORICAL  GEOLOGY 

is  the  evolution  of  the  horse.  In  the  earlier  Tertiary,  the  horse  was  a 
small  animal  about  the  size  of  a  fox  with  four  toes  on  the  front  feet  and 
three  toes  on  the  hind  feet.  During  many  generations  the  legs  became 
longer,  the  toes  were  reduced  to  hoofs,  the  teeth  became  better  adapted 
for  grazing  and  the  modern  horse  resulted.  Much  of  the  vegetation 
was  similar  to  modern  types. 

The  Quaternary  period  is  the  last  and  includes  the  present;  for 
example,  the  materials  deposited  by  a  stream  at  this  moment  are 
quaternary  in  age.  This  period  is  of  especial  interest  because  many  of 
our  soils  and  much  of  our  drainage  and  topography  have  been  developed 
during  this  period.  In  the  early  part  of  the  quaternary,  the  climate 
became  colder  and  the  great  ice  sheets  of  the  last  glacial  period  suc- 
cessively advanced  from  the  north,  both  in  North  America  and  Europe. 
It  will  be  remembered  that  this  glaciation  was  discussed  in  a  former 
chapter,  and,  from  a  viewpoint  of  soils,  perhaps  this  glaciation  was  the 
most  important  event  in  the  geological  history  of  North  America.  Also 
during  this  period,  the  great  fresh  water  lakes  of  Utah  and  Nevada  were 
in  existence,  Fig.  233. 


APPENDIX 


SOIL  MAPS 

The  following  soil  maps  illustrate  different  topics.  So  far  as  possible,  soil  maps 
have  been  selected  which  show  the  topography  or  for  which  there  are  corresponding 
topographic  maps.  The  (*)  indicates  that  the  topography  is  shown  on  the  soil  map. 
If  there  are  separate  topographic  maps  (sheets)  covering  the  soil  maps  wholly  or 
partly,  the  name  of  the  topographic  map  follows  that  of  the  soil  map.  The  soil 
maps,  unless  otherwise  stated,  are  issued  by  the  U.  S.  Bureau  of  Soils.  All  topo- 
graphic sheets,  folios,  monographs,  professional  papers,  bulletins  and  water-supply 
papers  are  issued  by  the  U.  S.  Geological  Survey  unless  otherwise  stated. 

CHAPTER  II 

Dikes  and  their  Soils. — *  San  Francisco  Bay  Region,  Cal.,  1914,  San  Francisco 
Folio;  Adam  Co.,  Pa.,  1904,  Gettysburg  and  Fairfield  sheets  (see  Fig.  18).  Soils 
Affected  by  Vulcanism. — Reconnoissance  Survey  of  the  Sacramento  Valley,  Cal., 
1913,  Marysville  Folio;  Trenton  Area,  N.  J.,  1902,  Trenton  Folio;  Fallen  Area, 
Nev.,  1909;  Austin  Area,  Tex.,  1904,  Austin  Folio;  Reconnoissance  Survey  of 
South-central  Texas,  1913,  Llano-Burnet  Folio. 

Residual  Soils  from  Folded  Rocks,  Mostly  Sedimentary. — Clay  Co.,  Ala.,  1915; 
Eltowah  Co.,  Ala.,  1908,  Gadsden  Folio;  Jefferson  Co.,  Ala.,  1908,  Birmingham  Folio; 
Blount  Co.,  Ala.,  1905,  Cullman  and  Springville  sheets.  Adjacent  regions  are  cov- 
ered by  the  Birmingham  and  Gadsden  Folios;  Fort  Payne  Area,  Ala.,  1903,  Fort 
Payne  sheet.  Adjacent  regions  are  covered  by  the  Gadsden  and  Stephenson  Folios; 
Chattoga  Co.,  Ga.,  1912,  Rome  and  Rmggold  Folios;  Walker  Co.,  Ga.,  1910,  Ring- 
gold  Folio;  Reconnoissance  Survey  of  Southeastern  Pa.,  1912;  Bedford  Co.,  Pa.,  1911, 
Bedford  and  Everett  sheets;  Reconnoissance  Survey  of  South-central  Pa.,  1910; 
Center  Co.,  Pa.,  1908,  Bellfonte  sheet  (in  part);  Johnstown  Area,  Pa.,  1907,  Johns- 
town Folio;  *  Lancaster  Co.,  Pa.,  1914;  Grainger  Co.,  Tenn.,  1906,  Maynardsville 
and  Morristown  Folios;  Greenville  Area,  Tenn.,  1904,  Greenville  Folio;  Pikesvile 
Area,  Tenn.,  1903,  Pikesville  Folio;  Frederick  Co.,  Va.,  1914;  Harrisonburg  Area, 
Va.,  1902,  Staunton  Folio;  Preston  Co.,  W.  Va.,  1912;  Jefferson,  Berkeley  and 
Morgan  Counties,  W.  Va.,  1918,  Pawpaw-Hancock  Folio. 

Soils  Affected  by  Faulting. — *  Montgomery  Co.,  Pa.,  1905,  Philadelphia  Folio; 
Austin  Area,  Tex.,  1904,  Austin  Folio;  San  Saba  Co.,  Tex.,  1917,  San  Saba  sheet, 
Llano-Burnett  Folio.  Reconnoissance  Survey  of  Southwest  Texas,  1911.  The 
Uvalde  Folio  shows  the  structure  of  part  of  the  area.  Washington  Co.,  Tex.,  1913. 

299 


300  APPENDIX 


CHAPTER  IV 

Residual  Soils  from  Sedimentary  Rocks. — Talledega  Co.,  Ala.,  1907;  Fayette- 
ville  Area,  Ark.,  1906,  Fayetteville  Folio;  *  Montgomery  Co.,  Kansas,  1913,  Inde- 
pendence Folio;  Madison  Co.,  Ky.,  1905,  Richmond  Folio;  Rockcastle  Co.,  Ky., 
1910,  Loudon  Folio;  Overton  Co.,  Ky.,  1908,  Standingstone  Folio;  Reconnoissance 
Survey  of  the  Ozark  Region  of  Missouri  and  Arkansas,  1916;  Ripley  Co.,  Mo.,  1915; 
Scotts  Bluff  Area,  Neb.,  1913,  Scotts  Bluff  Folio;  Muscogee  Co.,  Okla.,  1913,  Mus- 
cogee  Folio;  Lebanon  Area,  Pa.,  1901,  Lebanon  and  Hummelstown  sheets;  *  Lan- 
caster Co.,  Pa.,  1914;  Cambria  Co.,  Pa.,  1915,  Barnesboro-Patton,  Johnstown  and 
Ebensburg  Folios;  Taylor  Co.,  Tex.,  1918  (physiographic  map  included);  Mt. 
Pleasant  Area,  W.  Va.,  1910,  Charlestown  and  Huntington  Folios;  Clarksburg  Area, 
W.  Va.,  1910;  Kanawha  Co.,  W.  Va.,  1912,  Charlestown  Folio;  *  Preston  Co., 
W.  Va.,  1912;  *  Boone  Co.,  W.  Va.,  1913;  *  McDowell  and  Wyoming  Counties, 
W.  Va.,  1914,  Tazewell  and  Pocahontas  Folios;  *  Jefferson,  Berkeley  and  Morgan 
Counties,  W.  Va.,  1918,  Pawpaw-Hancock  and  Harper's  Ferry  Folios. 

Soils  from  Slates. — Chambers  Co.,  Ala.,  1909,  Wedowee  and  Grelina  sheets; 
Cleburne  Co.,  Ala.,  1913;  Randolph  Co.,  N.  C.,  1913;  Cabarrus  Co.,  N.  C.,  1910; 
Randolph  Co.,  N.  C.,  1913;  Granville  Co.,  N.  C.,  1910;  Fairneld  Co.,  S.  C.,  1911. 

Soils  from  Metamorphic  Rocks  and  from  Igneous  and  Sedimentary  Rocks. — 
Talledega  Co.,  Ala.,  1907;  Randolph  Co.,  Ala.,  1912,  Ashland  and  Wedowee  sheets; 
*  San  Francisco  Bay  Region,  Cal.,  1914,  San  Francisco  Folio;  Franklin  Co.,  Ga., 
1909,  Carnesville  sheet;  Johnson  Co.,  N.  C.,  1911;  Arisen  Co.,  N.  C.,  1915  (geological 
map  included);  Union  Co.,  N.  C.,  1914;  Richland  Co.,  N.  C.,  1916;  Chester  Co., 
Pa.,  1905,  Phoenixville,  Honeybrook,  Coatesville  and  Westchester  sheets.  The 
Philadelphia  Folio  covers  adjoining  territory,  Montgomery  Co.,  Pa.,  1905,  Philadel- 
phia Folio;  *  Lancaster  Co.,  Pa.,  1914;  Lehigh  Co.,  Pa.,  1912,  Hamburg,  Slating- 
ton,  Allentown  sheets;  Fairfax  and  Alexandria  Counties,  Va.,  1915,  Washington, 
D.  C.  Folio;  Louisa  Co.,  Va.,  1905;  *  Jefferson,  Berkeley  and  Morgan  Counties, 
W.  Va.,  1918,  Harpers  Ferry  Folio. 

Soils  from  Igneous  Rocks. — *  San  Francisco  Bay  Region,  Cal.,  1914,  San  Fran- 
cisco Folio;  Honey  Lake  Area,  Cal.,  1915;  Mecklenburg  Co.,  N.  C.,  1910,  part  of 
Charlotte  sheet;  York  Co.,  S.  C.,  1905,  Sharon  and  Kings  Mt.  sheets;  Oconee  Co., 
S.  C.,  1907. 

Inherited  Soils.— Cape  Girardeau  Co.,  Mo.,  1910  (see  Fig.  85);  Tishomingo  Area, 
Okla.,  1906,  Tishomingo  Folio  (see  Fig.  85). 

CHAPTER  V 

Soils  Affected  by  Wind  Work. — Reno  Co.,  Kans.,  1911,  Hutchinson  and  Lyons 
sheets;  North  Platte  Area,  Neb.,  1907;  Franklin  Co.,  Wash.,  1914,  physiographic 
map  included;  (see  Water  Supply  Paper  Nos.  118  and  316).  Loessial  Soils. — East 
Baton  Rouge  Parish,  La.,  1905;  Baton  Rouge  sheet;  East  and  West  Carroll  Parish, 
La.,  1908;  Lafayette  Parish,  La.,  1915;  Adams  Co.,  Miss.,  1910;  Wilkinson  Co., 
Miss.,  1913;  Granada  Co.,  Miss.,  1915;  Jackson  Co.,  Mo.,  1910,  Kansas  City,  Inde- 
pendence and  Harrisonville  sheets;  Franklin  Co.,  Mo.,  1913;  Cass  Co.,  Neb.,  1913; 
Douglass  Co.,  Neb.,  1913,  Fremont  sheet;  Nemaha  Co.,  Neb.,  1914;  Seward  Co., 
Neb.,  1914. 


APPENDIX  301 


CHAPTER  VI 

Soils  Modified  by  Ground  Water.—*  Ocala  Area,  Fla.,  1912;  Tattnal  Co.,  Ga., 
1914;  Easton  Area,  Md.,  1907;  Oktibebeha  Co.,  Miss.,  1907;  Duplin  Co.,  N.  C., 
1905;  Scotland  Co.,  N.  C.,  1909;  Columbus  Co.,  N.  C.,  1915. 

CHAPTER  VII 

Soils  Influenced  by  Head  Erosion  and  Soil  Creep. — Calhoun  Co.,  Ala.,  1908, 
Anniston  sheet;  Cobb  Co.,  Ga.,  1901,  Marietta  and  Cartersville  sheets;  Brown  Co., 
Kans.,  1905,  Hiawatha  and  Atchison  sheets;  *  Montgomery  Co.,  Kans.,  1913,  Inde- 
pendence Folio  (see  Fig.  165);  Rockcastle  Co.,  Ky.,  1910,  London  Folio;  Scotland 
Co.,  Mo.,  1905,  Medina  sheet;  *  Lancaster  Co.,  Pa.,  1914;  *  Cambria  Co.,  Pa., 
1915;  Jackson  Co.,  Tenn.,  1913,  Cowee  sheet.  Adjacent  regions  covered  by  the 
Nahantala  Folio;  Reconnoissance  Survey  of  the  Panhandle  of  Texas,  1910  (see  Fig. 
110);  Upshur  Co.,  W.  Va.,  1908,  Buckhannon  Folio;  *  Boone  Co.,  W.  Va.,  1913; 

*  Logan  and  Miller  Counties,  W.  Va.,  1913;   *  Lewis  and  Gilmer  Counties,  W.  Va., 
1915,  Buckhannon  Folio.   Bad  Lands. — McKenzie  Area,  N.  D.,  1907;  Morton  Area, 
N.  D.,  1907. 

CHAPTER  VIII 

Braided  Streams. — Grand  Island  Area,  Neb.,  1903;  Kearney  Area,  Neb.,  1904. 
Flood  Plain  Soils. — Miller  Co.,  Ark.,  1903;  Mississippi  Co.,  Ark.,  1914;  Jefferson 
Co.,  Ark.,  1915;  *  Redding  Area,  Cal.,  1907;  St.  Clair  Co.,  111.,  1902;  New  Orleans 
Area,  La.,  1903,  New  Orleans  sheet;  Concordia  Parish,  La.,  1910;  East  and  West 
Carroll  Parish,  La.,  1908;  Rapides  Parish,  La.,  1916;  Holmes  Co,,  Miss.,  1908; 
Adams  Co.,  Miss.,  1910;  Coahoma  Co.,  Miss.,  1915;  O'Fallon  Area,  Mo.,  1904, 
O'Fallon  sheet;  Atchison  Co.,  Mo.,  1909;  Platte  Co.,  Mo.,  1912,  Kansas  City  sheet; 
Grundy  Co.,  Mo.,  1914;  Sarpy  Co.,  1905;  Meigs  Co.,  Ohio,  1906,  Keno  and  Ravens- 
worth  sheets;  Oklahoma  Co.,  Okla.,  1906;  Bryan  Co.,  Okla.,  1914. 

Terrace  Soils.— Tazewell  Co.,  111.,  1902;  La  Salle  Co.,  111.,  Soil  Report  No.  5, 
Illinois  Ag.  Exp.  Station,  1913,  La  Salle,  Ottawa  and  Marseilles  sheets;  *  Amherst 
Area,  Mass.,  1903;  *  Springfield  Area,  Mass.,  1903;  Scotts  Bluff  Co.,  Neb.,  1913, 
Scotts  Bluff  Folio;  Douglas  Co.,  Neb.,  1913,  Fremont  sheet;  Dodge  Co.,  Neb.,  1914; 
Merrimac  Co.,  N.  H.,  1906;  *  Oneida  Co.,  N.  Y.,  1913;  Clinton  Co.,  N.  Y.,  1914; 

*  Stark  Co.,  Ohio,  1913;  Muscogee  Co.,  Okla.,  1913,  Muscogee  Folio;  Bastrop  Area, 
Tex.,  1907,  Bastrop  sheet;  Huntington  Area,  W.  Va.,  1913,  Huntington  and  Charles- 
ton Folios;    Clarksburg  Area,  W.  Va.,  1912;   Kanawha  Co.,  W.  Va.,  1912;    *  Mt. 
Pleasant  Area,  W.  Va.,  1910;    *  Point  Pleasant  Area,  W.  Va.,  1910;    Parkersburg 
Area,  W.  Va.,  1908;   *  Middlebourne  Area,  W.  Va.,  1907;   *  Wheeling  Area,  W.  Va., 
1906;    *  McDowell  and  Wyoming  Counties,  W.  Va.,  1914;    *  Jefferson,  Berkeley 
and  Morgan  Counties,  W.  Va.,  1918. 

Delta  Soils. — Indio  and  Imperial  Areas,  Cal.,  1903  (Salton  Sink);  San  Jose  Area, 
Cal.,  1903;  Ventura  Area,  Cal.,  1901;  Stockton  Area,  Cal.,  1905;  *  San  Francisco 
Bay  Region,  Cal.,  1914;  Reconnoissance  Survey  of  South  Texas,  1909  (Rio  Grande 
delta).  Alluvial  Fan  Soils. — San  Bernardino  Valley,  Cal.,  1904,  Pomona  and 
Cucamonga  sheets;  Fresno  Area,  Cal.,  1912;  San  Jose  Area,  Cal.,  1903,  San  Jose 


302  APPENDIX 

sheet;    Bakersville  Area,   Cal.,   1904;     *  San  Fernando  Valley  Area,   Cal.,   1915; 

*  Riverside  Area,  Cal.,  1915;    *  Pasadena  Area,  Cal.,  1915,  Bitterroot  Valley  Area, 
Mont.,  1914;  Cache  Valley  Area,  Utah,  1913. 

CHAPTER  IX 

Colluvial  Soils.— Marysville  Area,  Cal.,  1909;  St.  Clair  Co.,  111.,  1912,  Belleville 
sheet;  *  Montgomery  Co.,  Kans.,  1913;  *  Cambria  Co.,  Pa.,  1915,  Barnesboro- 
Patton,  Johnstown  and  Ebensburg  Folios;  Albermarle  Area,  Va.,  1902;  Bedford 
Area,  Va.,  1901. 

CHAPTER  X 

Morainic  Soils. — *  Portage  Co.,  Ohio,  1914;  Pontiac  Area,  Mich.,  1903;  Cass 
Co.,  Mich.,  1905;  Oxford  Area,  Mich.,  1905;  *  Ramsey  Co.,  Minn.,  1914,  Minneap- 
olis, St.  Paul  Folio;  Racine  Co.,  Wis.,  1906  (see  Alden,  Professional  Paper  No.  34). 
Fond  Du  Lac  Co.,  Wis.,  1911,  part  of  area  covered  by  Fond  du  Lac  sheet;  Waukesha 
Co.,  Wis.,  1910,  Milwaukee  Folio.  Drumlins. — *  Auburn,  N.  Y.,  1904;  *  Lyons 
Area,  N.  Y.,  1912.  Glacial  Soils  Affected  by  Underlying  Rocks. — *  New  London 
County,  Conn.,  1912;  *  Plymouth  Co.,  Mass.,  1911;  Caribou  Area,  Me.,  1908; 

*  Cumberland  Co.,  Me.,  1915  (see  Monograph  34);  *  Oneida  Co.,  N.  Y.,  1913;  Clin- 
ton Co.,  N.  Y.,  1914;  Jefferson  Co.,  N.  Y.,  1911;  Reconnoissance  Survey  of  North- 
eastern Wisconsin,   1913;    Marinette  Co.,  Wis.,   1909;    Portage  Co.,  Wis.,   1905; 
Wood  Co.,  Wis.,  1915  (For  Portage  and  Wood  Counties,  see  Geology  of  North 
Central  Wisconsin  by  Samuel  Weidman,  Bull.  16,  Wis.  Geological  and  Natural  His- 
tory Survey,  1907).    Relations  between  Glacial  Soils  and  the  Strikes  of  Under- 
lying Rocks.—*  Orange  Co.,  N.  Y.,  1912;    *  Sussex  Co.,  N.  J.,  1911;  Different  Drifts 
Mapped  by  Soils.—*  Dane  Co.,  Wis.,  1913;  Rice  Co.,  Minn.,  1909.     Maps  Showing 
Adjacent  Glaciated  and  Unglaciated  Soils. — Stark  Co.,  Ohio,  1913;  Reconnoissance 
Survey  of  Ohio,  1912  (geological  map  included);    *  Dane  Co.,  Wis.,  1913. 

Soils  of  Outwash  Plains.— Long  Island  Area,  N.  Y.,  1903  (see  M.  L.  Fuller,  Pro- 
fessional Paper  No.  82);  Jefferson  Co.,  Wis.,  1912,  Eagle  and  Whitewater  sheets. 
Eskers. — Orono  Area,  Me.,  1909.  Miscellaneous  Fluvio-glacial  Soils. — *Cumberland 
Co.,  Me.,  1914;  *  Bitterroot  Valley  Area,  Mont.,  1914;  *  Oneida  Co.,  N.  Y.,  1913. 
Miscellaneous  Glacial  Soils. — *  Cumberland  Co.,  Me.,  1915  (see  Monograph  34); 

*  Ramsey  Co.,  Minn.,  1914,  Minneapolis,  *  St.  Paul  Folio;   Seward  Co.,  Neb.,  1914; 

*  Chautauqua  Co.,  N.  Y.,  1914;   *  Schoharie  Co.,  N.  Y.,  1915;   *  Clinton  Co.,  N.  Y., 
1914;   Ohio  Reconnoissance  Survey,  1912;    *  Rhode  Island  Survey,  1904;   Soils  of 
North  Central  Wisconsin  by  Samuel  Weidman,  Bull.  No.  11,  Wis.  Geological  and 
Natural  History  Survey,   1903;    Reconnoissance  Survey  of  Part  of  Northeastern 
Wisconsin  by  Weidman,  Hall  and  Musback,  Bull.  23,  Wis.  Geological  and  Natural 
History  Survey,  1911;  see  also  the  soil  reports  of  the  Experiment  Stations  of  Illinois 
and  Iowa. 

CHAPTER  XI 

Filled  and  Partly  Filled  Lakes.— Hillsboro  Co.,  Fla.,  1916;  East  and  West  Carroll 
Parishes,  La.,  1909  (see  Fig.  132);  Concordia  Parish,  La.,  1911;  *  Plymouth  Co., 
Mass.,  1911;  Cass  Co.,  Mich.,  1906;  *  Orange  Co.,  N.  Y.,  1912;  *  Clinton  Co., 


APPENDIX  303 

N.  Y.,  1915;  Columbus  Co.,  N.  C.,  1915;  Richland  Co.,  N.  D.,  1908;  *  Dane  Co., 
Wis.,  1913.  Lake  Bottom  and  Shore  Soils.— Fort  Wayne,  Ind.,  1908;  Wells  Co., 
Ii  d.,  1915  (see  Monograph  41,  plate  11);  Crookston  Co.,  Minn.,  1906  (see  Mono- 
graph 25);  Pennington  Co.,  Minn.,  1914  (see  Monograph  25);  Westfield  Area,  N.  Y., 
1901;  Vergennes  Area,  Vt.-N.  Y.,  1904;  Niagara  Co.,  N.  Y.,  1906,  Niagara  Folio; 
Washington  Co.,  N.  Y.,  1909;  *  Jefferson  Co.,  N.  Y.,  1911;  *  Monroe  Co.,  N.  Y., 
1910;  *  Chautauqua  Co.,  N.  Y.,  1914;  Lake  Mattamuskeet  Area,  N.  C.,  1909; 
*  Fargo  Area,  N.  D.,  1903,  Casselton-Fargo  Folio;  Cleveland  Area,  Ohio,  1905, 
Cleveland  and  Berea  sheets;  Ohio  Reconnoissance  Survey,  1912;  Erie  Co.,  Pa., 
1910;  Provo  and  Goshen  Areas,  Utah,  1903,  Salt  Lake  sheet;  Cache  Valley  Area, 
Utah,  1913.  Glacial  Deltas.— Tompkins  Co.,  N.  Y.,  1905,  Watkins  Glen  Folio. 

CHAPTER  XII 

Ocean  Shore  Lines.— Indian  River,  Fla.,  1913;  Long  Island  Area,  N.  Y.,  1903; 
Anne  Arundel  Co.,  Md.,  1909;  Charleston  Area,  S.  C.,  1904;  Reconnoissance  Survey 
of  South  West  Texas,  1911;  Jefferson  Co.,  Tex.,  1913.  Tidal  Marshes.— *  San 
Francisco  Bay  Region,  Gal.,  1914;  *  New  Castle  Co.,  Del,  1915;  Hernando  Co., 
Fla.,  1914;  Hillsboro  Co.,  Fla.,  1916. 


LOCALITY   INDEX 


Adirondacks,  N.  Y.,  293 

Agassiz,  Lake,  225,  226,  241,  244,  245,  247 

Alabama,  46,  59,  87,  265,  266,  294,  296 

Alaska,  169,  183,  193,  "199,  213,  297 

Albany,  N.  Y.,  263 

Allegheny  Plateau,  179,  210 

Allegheny  River,  223 

Alps,  59 

American  Fork  River,  170 

Andes,  Argentina,  174 

Antarctic,  192 

Appalachian  Plateau  Region,  287,  288, 

289,  294 

Appalachian  Ridge  Belt,  39,  54,  87,  89 
Appalachians,  50,  141,  182,  295,  296 
Argentina,  174 
Arizona,  34,  65,  111,  296 
Arkansas,  90,  287,  288 
Atlantic  City,  N.  J.,  260 
Austin,  Texas,  30 

B 

"  Balcones,"  Texas,  64 

Baltimore,  Md.,  263 

Black  Belt  of  Ala.,  and  Miss.,  89 

Black  Prairie,  Texas,  89 

"  Blue  Grass"  Basin,  Ky.,  and  Tenn.,  63, 

90,  103 

Blue  Mt.,  Pa.,  62 
Blue  Ridge  Mountains,  N.  C.,  138,  Va., 

187 

Bonneville,  Lake,  169,  240,  249 
Boston,  Mass.,  202 
Brahmaputra  River,  169 
Brazos  River,  165 


Cairo,  111.,  160 

California,  65,  123,  174,  175,  176,  177, 

-  261,  291 

Canada,  157,  193,  198,  221,  244,  293,  294, 

295 

Cascade  Mountains,  Wash.,  291 
Castle  Rock,  Nebraska,  33 
Chesapeake  Bay,  166 
Chicago,  Lake,  223,  227 
Chili,  283 

China,  116,  117,  118,  143 
Coast  Range  Mts.,  Cal.,  177,  291 
Coastal  Plain,  45,  46,  64,  71,  104,  105, 

127,  132,  165,  166,  252,  265,  266,  286, 

297 

Colorado,  27,  28,  34,  46,  188 
Colorado  Canyon,  138 
Colorado  River,  133,  136,  149,  165 
Columbia  Lava  Plateau,  Wash.,  36, 101, 

297 

Commencement  Bay,  166 
Connecticut,  56,  156,  195,  210,  296 
Connecticut  River,  156 
Connecticut  Valley,  290 
Cordilleran  Mts.,  59 
Cumberland  Plateau,  179 
Cumberland- Allegheny  Plateau,  288,  289 
Cumberland  Valley,  60 


Danube  River,  146 
Deccan  Plateau,  India,  37 
Drummond,  Lake,  252 
Duluth,  Lake,  227 


305 


306 


LOCALITY  INDEX 


E 

Edwards  Plateau,  Texas,  64 
Erie,  Lake,  227,  240 
Europe,  118,  191 
Everglades,  Fla.,  254,  255 


Florida,  254,  275,  278,  279,  297 
Fort  Wayne,  Ind.,  227 


Galveston,  Texas,  242,  260 

Ganges  River,  169 

"  Garden  of  the  Gods,"  Colo.,  46 

Geneva,  Lake,  240 

Georgia,  87 

Gettysburg,  Pa.,  28 

"  Grand  Coulee,"  Wash.,  36 

Great  Basin,  169,  170,  187,  249,  290 

Great  Dismal  Swamp,  Va.,  252 

Great  Lakes,  205,  226,  227 

Great  Plains,  46,  290,  297 

Great  Salt  Lake,  249 

Great  Valley,  89 

Green  Mts.,  Mass.,  52 

Greenland,  192,  197,  198 

Gulf  of  California,  291 

H 

Hawaiian  Islands,  35 

Heligoland,  260 

High  Plains,  120,  137,  138,  174,  176 

Highland  Rim,  Tenn.,  63 

Himalaya  Mts.,  174 

Holland,  168 

Holmes  Island,  Mo.,  151 

Hudson  Bay,  226 

Hudson  Valley,  N.  Y.,  263 


Iceland,  36 

Idaho,  25,  36 

Illinois,  113,  117,  118,  153,  207,  228,  296 

Illinois  River,  224,  227 

India,  169,  174 

Indiana,  109,  113,  118,  295 


Iowa,  113,  117,  118,  207,  222,  232,  233 
Irawaddy  River,  146 
Italy,  107 


Jefferson  River  Valley,  Montana,  34 

K 

Kansas,  126,  155,  160,  180,  183,  184,  232 

Kansas  River,  154 

Kentucky,  63,  90,  103,  123,  210,  288 

Keokuk,  Iowa,  222 

Kilauea,  35 

Kittatinny  Valley,  N.  J.,  59 

Klamath  Mts.,  Ore.,  and  Cal.,  291 

Krakatoa,  East  Indies,  35 


Lahontan,  Lake,  249 
Lebanon  Valley,  Pa.,  59 
Liverpool,  Eng.,  263 
London,  Eng.,  263 
Long  Island,  N.  Y.,  215,  261 
Louisiana,  112,  113,  116,  117,  126,  152, 
154,  159,  160,  161,  240 

M 

Maine,  214 

Mammoth  Cave,  Ky.,  123 
Mar  River,  Europe,  147 
Maryland,  59 
Massachusetts,  52,  209 
Maumee,  Lake,  223,  224,  227,  228,  246 
Mauna  Loa,  35 
Meuse  River,  169 

Michigan,  110,  111,  206,  218,  243,  258 
Michigan,  Lake,  223,  227,  240 
Middle  West,  200,  207 
Minneapolis,  Minn.,  222 
Minnesota,  244 
Minnesota  River,  226 
Mississippi,  116 

Mississippi  Basin,  108,  114,  137,  165,  295 
Mississippi  River,  146,  153,  161 
Mississippi  River  Delta,  166,  167,  169, 
170,  222 


LOCALITY  INDEX 


307 


Missouri,  85,  90,  113,  151,  158,  178,  180, 

207,  232,  296 
Missouri  River,  157,  159 
Montana,  34,  239,  277 
Mt.  jEtna,  Sicily,  33 

N 

Narrangansett  Basin,  209 

Nebraska,  33,  34,  108,  117,  118,  143,  232 

Nevada,  249,  298 

New  England,  166,  200,  236,  289,  293 

New  Jersey,  15,  59,  201,  211,  236,  260, 

265,  281,  296 
New  Mexico,  35,  65,  165 
New  Orleans,  168 
New  York,  30,  46,  205,  215,  242,  245, 

252,  261,  263,  265,  289,  294,  295 
Niagara  Falls,  135 
Niagara  River,  135,  166,  240 
Nile  River,  146,  149 
North  Carolina,  49,  94,  98,  184,  252,  257, 

287,  296 

North  Dakota,  143,  247,  251 
Nova  Scotia,  59 

O 

Ohio,  207,  228,  235,  236,  243,  246,  295 

Ohio  River,  139,  160 

Okechobee,  Lake,  254 

Oklahoma,  104,  287,  296 

Oregon,  25,  36,  167 

Osage  Valley,  178,  179 

Ozark  Mountains,  Mo.,  85,  105,  187,  288 


Pacific  Coast,  291 

Palisades  of  Hudson  River,  30 

Panama  Canal,  181 

Payallup  River,  166 

Pennsylvania,  28,  46,  53,  59,  60,  62,  92, 

101,  236,  289,  295,  296 
Philadelphia,  Pa.,  263 
Pilot  Knob,  Austin,  Texas,  30,  31 
Po  River,  146 
Pontchartrain,  Lake,  239 
Potomac  River,  146. 
Puget  Sound,  167,  291 


Red  River,  165 

Red  River  of  the  North,  178,  226 
Rhine  River,  169 
Rhode  Island,  209 
Rhone  River,  240 
Rio  Grande  Delta,  171 
Rio  Grande  River,  146 
Rochester,  N.  Y.,  222 
Rock  River,  Wis.,  219 
Rocky  Mts.,  46,  50,  132,  174,  176,  271, 
290,  293,  297 


Sacramento  River  Cal.,  155,  156,  291 

Spmbre  River,  169 

Sandy  Hook,  N.  J.,  242 

San  Francisco,  Cal.,  291 

San  Joaquin  River,  291 

Scheldt  River,  169 

Seward,  Alaska,  169 

Shenandoah  Valley,  Va.,  59 

Sheyenne  River,  245 

Sicily,  33 

Sierra  Nevadas,  65,  174,  176,  291 

Snake  River,  249. 

South  Carolina,  252,  262 

South  Dakota,  143 

South  Mt.,  Pa.,  60 

Spanish  Peaks  Region,  Colo.,  27,  28 

Stassfurt  Region,  Germany,  282 

St.  Croix  River,  227 

St.  Lawrence  River,  135,  227 

Stephens  Mt.,  British  Columbia,  198 

Sundance,  Mt.,  Wyo.,  31 

Superior,  Lake,  227,  294 

Susquehanna  River,  166 


Tennessee,  101,  123 

Appalachian  ridges  of,  87 

Blue  grass  region  of,  63,  90 

Highland  rim,  63,  288 

Phosphates,  275,  276,  294 

Valley,  101,  123 
Texas,  30,  31,  63,  64,  65,  71,  137,  165. 

242,  260,  265,  266,  296,  297 
Trinity  River,  165 


308 


LOCALITY  INDEX 


U 

Utah,  170,  220,  249,  280,  296,  298 

V 

Vermont,  52,  53 

Vesuvius,  35,  101 

Virginia,  53,  195,  210,  236,  252,  281 


W 


Wabash  River,  227 
Wales,  53 


Washington,  25,  36,  72,  162,  166 

West  Virginia,  186,  295 

Whiteside  Mt.,  Southern  Appalachians, 

182 

Willamette  River  Valley,  Ore.,  291 
Winnipeg,  Lake,  226 
Wisconsin,  85,  113,  200,  205,  207,  208, 

209,  210,  211,  214,  219,  253,  294 
Wyoming,  31,  143,  280,  281 


Yucatan,  265 


AUTHORS  INDEX 


Alden,  W.  C.,  200,  208,  209,  214,  218, 

219,  236,  242 
Alway,  F.  J.,  117 
Anderson,  J.  G.,  189 
Atwood,  W.  W.,  196,  220 

B 

Babb,  C.  C.,  146 

Barrett,  E.,  237 

Bell,  N.  R.  E.,  130 

Bennett,  H.  H.,  90,  102,  178,  252 

Blair,  A.  W.,  237 

Blish,  W.  L.,  117 

Bowman,  L,  130,  292 

Brown,  P.  E.,  117 

Buckman,  H.  O.,  80,  102,  130 


Chamberlin,  T.  C.,  37, 102,  107, 119, 132, 

177,  209,  231,  236,  271 
Clarke,  F.  W.,  17,  42,  94 
Coffey,  G.  N.,  118,  235,  236,  292 
Conn,  H.  W.,  81 


Dachnowski,  A.,  253 

Dale,  T.  N.,  200 

Daly,  R.  A.,  18,  21,  22,  23,  24,  26,  273 

Dana,  E.  S.,  15 

Darton,  N.  H.,  31,  33,  258 

Davis,  W.  M.,  139,  180 

E 

Edgerton,  C.  W.,  81 
Emerson,  B.  F.,  37 


Fenneman,  N.  M.,  253 
Fippin,  E.  O.,  80,  102,  130 
Free,  E.  E.,  119 

Fry,  Wm.  H.,  75,  89,  93,  100,  114 
Fuller,  M.  L.,  132 

G 

Geib,  W.  J.,  207 

Geikie,  A.,  119 

Geikie,  J.,  102,  119,  132,  191,  236,  271 

Gilbert,  G.  H.,  258,  271 

Glenn,  L.  C.,  177,  182 

Grabeau,  A.  W.,  102,  119,  147,  177,  189, 

258,  262,  284 
Gregory,  H.  E.,  Ill 

H 

Hayes,  C.  W.,  65,  275,  276,  284 
Hilgard,  E.  W.,  72,  73,  77,   102,  125, 

130,  284 

Hill,  J.  M.,  28,  64 

Hobbs,  W.  H.,  37,  189, 194,  236,  258,  271 
Hopkins,  C.  G.,  117,  237 
Hovey,  E.  O.,  132 
Howe,  E.,  188,  189 


Isham,  R.  M.,  117 


Jenning,  H.,  237 

Johnson,  D.  W.,  137,  138,  174,  178,  272 

Jones,  S.  C.,  102 

K 

Kemp,  J.  F.,  26,  46 
King,  F.  H.,  126,  132 


309 


310 


AUTHORS  INDEX 


Landes,  H.,  36 

Lapham,  J.  E.,  119,  177,  237 

Leverett,  F.,  119,  206,  224,  227,  228,  229, 

237,  243,  244 
Lipman,  J.  G.,  81 
Logan,  W.  N.,  102 
Lyon,  T.  L.,  80,  102,  130 

M 

Marbut,  C.  F.,  65,  71,  95,  119,  178,  237 
Martin,  L.,  102,  119,  177,  180,  194,  237, 

258,  272 

Matson,  G.  C.,  278,  279,  284 
Matthes,  F.  E.,  237 

McCaughey,  W.  J.,  75,  89,  93,  100,  114 
McGee,  W.  J.,  177,  292 
Meinzer,  O.  E.,  35 
Merrill,  G.  P.,  15,  26,  46,  72,  88,  102, 

119,  177,  189,237,257,258 
Moffit,  F.  H.,  183 


Paige,  S.,  76 
Palmer,  W.  C.,  247 
Parks,  E.  M.,  239 
Pettit,  J.  S.,  117 
Pettot,  J.  H.,  237 
Pirson,  L.  V.,  15,  26 
Plummer,  J.  K,  98 
Pumpelly,  R.,  85 


Ransome,  F.  L.,  65 
Ries,  H.,  15,  132,  189,  272,  274,  284 
Robinson,  H.  H.,  34 
Russell,  I.  C.,  37,  145,  177,  194,  218,  237, 
258 


Salisbury,  R.  D.,  37,  102,  107,  119,  132, 
177,  209,  231,  236,  237,  258,  271 

Sanford,  F.  H.,  110,  111 

Schuchert,  C.,  136 

Selecter,  I.,  117 

Sellards,  E.  H.,  102,  254,  255,  258,  284 

Shaler,  N.  S.,  37,  48,  103,  106,  119,  132, 
189,  254,  258,  262,  272 

Shaw,  C.  F.,  172,  177 

Shaw,  E.  W.,  171 

Stokes,  H.  N.,  40,  44,  273 


Tarr,  R.  S.,  102,  119,  177,  180,  194,  205, 

213,  237,  258,  272 
Taylor,  F.  B.,  224,  227,  237 
Thomas,  B.  F.,  177,  272 
Todd,  J.  E.,  150 
Transeau,  E.  N.,  256 

U 

Udden,  J.  A.,  108,  119 
Ulrich,  E.  O.,  276 
Upham,  W.,  225,  226,  237,  245,  258 


Van  Hise,  C.  R.,  69,  70,  71,  72,  132 
Von  Engelin,  O.  D.,  245 

W 

Watson,  T.  L.,  15,  97,  132,  189,  272 

Watt,  D.,  177,  272 

Weed,  W.  H.,  77 

Whitbeck,  R.  H.,  210,  235 

Whitman,  F.  L.,  85 

Willis,  B.,  65,  117 

Woodworth,  J.  B.,  48 


SUBJECT   INDEX 


Abrasion,  by  wind  work,  110 

streams,  146 

Accessory  minerals,  20 

Agate,  11 

Alabaster,  9 

Albite,  13 

Algae,  135 

Alkali,  129 

Alluvial,  deposits  in  channels,  150 

—  fans  and  cones,  171 

—  terraces,  161 

—  terrace  soils,  163 

—  variability  of  soils,  155 
Alluviation,  147 
Amphibians,  296 
Analyses  of  biotite  granite,  21 
a  diorite,  23 

a  diabase,  100 

—  - —  earth's  crust,  5 
a  gabbro,  24 

—  glacial  lake  clay,  203 

granite  and  its  residual  clay,  £7 

igneous  rocks,  17 

-  Illinoian  drift,  230 

—  Kaolinite,  42 
limestones,  44 

limestone  and  its  residual  clay,  88 

materials    transported    by    rivers, 

135 

prairie  lands,  257 

residual  clay,  203 

sandstone,  40 

shales,  42 

swamp  soils  and  bottom  lands,  257 

a  syenite,  22 

Wisconsin  drift,  230 


Anorthite,  13 
Anticline  defined,  59 
Apatite,  8,  273 

—  hardness  of,  6 

—  percentage  in  Igneous  rocks,  17 
Aragonite,  9 

Artesian  water  in  sandstones  and  con- 
glomerates, 120 
Augite,  13 

B 

Barrier  beaches,  242,  260 
Bars,  242 
Basalt,  25 

—  soils,  100 
Bathgliths,  32 

Bedding  planes  of  strata,  45 
Berks  soil  series,  59 
Biotite,  13 

—  percentage  in  Igneous  Rocks,  17 
Bituminous  coal,  295 

Block  mountains  defined,  65 
Bog  lima,  255 
Bombs,  volcanic,  33 
Bosses,  31 
Breccia,  41 

C 

Calcite,  8 

—  hardness  of,  6 

—  basic  mineral  of  limestone,  42 
Cambrian  period,  293,  294 
Carbonation  as  a  weathering  process,  68 
Carboniferous  system,  295 
Catchment  area,  131 

Caverns  due  to  solution,  123 
Cecil  soils  of  N.  C.,  29 

—  mineral  composition,  98 


311 


312 


SUBJECT  INDEX 


Cenozoic  era,  293,  297 
Chalcedony,  11 
Chalk,  44 
Chert,  11,  44 
Chester  series,  98 
Chickamauga  limestone,  88 
Cirques,  222 
Clastic  Rocks,  37 

Agents  involved  in  formation  of, 

37 

carboniferous,  295 

—  chalk,  44 

chert,  44 

conglomerates,  40 

dolomites,  44 

limestones,  42 

references  on,  46 

sandstone,  39 

shales,  41 

structure  of,  45 

Clay,  residual  from  limestone,  87,  295 
Cleavage  of  minerals,  7 
Clinton  formation,  294 
Coal  beds,  295 
Coasts,  262 

—  depressed,  263 

—  elevated,  263 
Coast  plains,  264 

boundaries  of,  266 

erosion  of,  266 

Lafayette  and    Columbia    forma- 
tions, 267 

—  origin,    265 

Colluvial  soils,  formation  of,  181 
Color  of  minerals,  6 

as  affecting  disintegration,  75 

Columbia  formation,  267 

origin  of,  268 

references  on,  269 

Comanchean    Cretaceous    period,    293, 

296,  297 

Conasauge  shale,  88 
Concretions,  128,  129 
Conglomerates,  40 
Contour  plowing,  142 
Coquina,  43 
Corals,  43,  270 
Corrasion  by  rivers,  135 


Corrosion  by  rivers,  134 

Corundum,  53 

Cretaceous  period,  14,  46,  293,  294,  297 

Crystal  form  of  minerals,  8 

Culver  stony  loam,  212 

Cumulose  soils,  250 

content  of,  256 

D 

Decomposition  defined,  67 
• —  processes  of,  68 
Dekalb  series,  93 
Deltas,  classes  of,  169 
— -  formation  of,  166 

—  growth  of,  167 

—  materials  of,  170 

—  as  shore  features,  245 

—  soils  of,  171 

Deposition,  by  ground  water,  124,  127 

-  —  rivers,  147,  148,  149 
Detritus,  182 
Devonian  rocks,  276 

period,  293,  295 

Dikes,  27 

Diorite,  chemical  composition  of,  23,  100 

—  mineralogical  composition  of,  23 

—  soils,  99 
Dip,  defined,  58 
Disintegration,  denned,  67 

—  processes  of,  73 

—  soils  due  to,  74 
Displacement  applied  to  faults,  64 
Dolomite,  9 

Dolomitic  limestone,  44 

Dover  soils,  212 

Drift,  composition  of,  202 

—  definition  of,  201 

—  thickness  of,  201 
Drumlins,  208 
Dunes,  fixing  of,  110 

—  formation  of,  109 
Durham  series,  98 
Dutchess  soils,  212 


Earth's  crust,  analysis  of,  5 
Eocene,  46 


SUBJECT  INDEX 


313 


Eolian  soils,  107 

Erosion,  by  rivers,  134,  156 

—  by  waves,  241,  259,  260 

—  cycle  of,  178 

—  defined,  67 

—  features  of  ice,  221 
-head,  137,  184 

—  ice,  196 

—  of  the  coastal  plain,  266 

—  remedies  for,  143 
Eruptions,  33,  34,  35,  36 
Escarpment,  137 
Eskers,  217 

Essential  minerals,  20 
Exfoliation,  73 
Extrusive  rocks,  17,  27 


Faults,  defined,  63 

—  effects  of,  64 

—  hade  of,  64 

—  heave,  64 

—  references  on,  66 

—  throw,  64 
Feldspars,  12 

—  essential  mineral  in  granite,  20 

—  hi  schist  soils,  101 

—  percentage  in  igneous  rocks,  17 

—  plagioclase,  13 
Felsites,  25 
Felsitic  texture,  25 
Ferromagnesian  minerals,  15 
Fiords,  221 

Fissure  flows,  32 

Flint,  11 

Flood  plains  and  valleys,  156 

meanders,  157 

Mississippi,  160 

origin  of,  151 

settlement  of,  159 

soils  of,  153 

Florissant  beds,  34 
Folding,  anticline,  59 

—  structures  due  to,  57 

—  syncline,  59 

—  topography  produced  by,  61 
Foliated  metamorphic  rocks,  49 
kinds  of,  52 


Formation,  defined,  45 
Fluids  as  metamorphic  agents,  48 
Fluorite,  hardness  of,  6 
Fluvio-glacial  deposits,  212 

kames  and  eskers,  217 

outwash  plains,  213 

valley  train,  216 

Fossils,  294 

Fracture,  of  minerals,  7 

Fresno  loam,  75 

Frost,  effect  on  rocks,  76 


Gabbro,  23 

—  chemical  composition,  24 

—  mineralogical  composition,  24 
Garnets,  53 

—  schist  soils,  101 

Gases  in  metamorphism,  48 
Glacial  deposits,  buried  valleys,  202 
t by  water,  212 

—  drumlins,  208 

eskers,  217 

kames,  217 

moraines,  203 

nature  of,  202 

outwash  plain,  213 

valley  tram,  216 

—  drift,  201 

composition  of,  202 

—  lakes  in,  223 

thickness  of,  201 

topography  of,  219 

—  lakes,  abandoned,  224 

marginal  glacial,  223 

morainic,  238 

references  on,  237 

—  rock  basin,  238 

—  soils  of,  224 

-  period,  162,  191,  228 
causes  of,  236 

—  references  on,  234,  236 
stages  in,  231 

—  soils,  209,  229 

references  on,  237 

value  of,  234 

Glaciers,  advancing,  193 

—  conditions  of  formation,  192 


314 


SUBJECT  INDEX 


Glaciers,  continental,  192 

—  deposition  by,  200 

—  drainage  changes  due  to,  219 

—  erosion  by,  194,  221 

—  fluvio-glacial  deposits,  212 

—  mountain,  191 

—  rapidity  of  movement,  193 

—  references  on,  194,  236,  237 

—  transportation  by,  197 
Glassy  texture,  19 
Glauconite,  14 

—  as  potash  fertilizer,  281 
Gloucester  soils,  212 
Gneiss,  52 

Granite,  20 

—  binary  granites,  20 

—  chemical  composition,  21 

—  mineralogical  composition,  21 

—  notable  regions  of,  98 

—  pegmatite,  21 

—  weathering  of,  95 
Granitoid  texture,  18 

Ground  water,  deposition  by,  124 

mineral  veins  due  to,  124 

movements  of,  122 

references,  130 

relation    rock    material    and    dis- 
solved matter,  122 

solution  by,  122 

source,  120 

springs,  131 

water  table,  120 

wells,  drainage  by,  131 

work  of,  122 

Gypsum,  9,  296 

—  as  a  fertilizer,  284 

—  hardness  of,  6 

H 

Hade  of  fault,  64 
Hagerstown  soil  series,  59 
Halite,  9,  297 
Hanging  valleys,  221 
Hardness,  of  minerals,  6 
Hardpan,  129 

Heat  as  a  metamorphic  agent,  47 
Heave  applied  to  faults,  64 
Hematite,  10,  128 


Hornblende,  13 

—  in  schist  soils,  101 
Humus,  defined,  79 

—  distribution  of,  79 

—  weathering  effects  of,  80 
Hydration  as  a  weathering  process,  70 


I 

Ice,  erosion  work  of,  194 

Igneous  rocks,  accessory  minerals,  20 

analysis  of,  17 

—  basalt,  25 

-  batholiths,  27 

—  bosses,  27 

—  chemical    composition    of,  21,  22, 

23,  24 

classification  of,  16,  26 

defined,  17 

—  description  of,  20 

—  dikes,  27 

—  diorite,  23 

essential  minerals  in,  20 

extrusive,  17 

—  felsite,  25 
gabbro,  23 

—  glassy,  19,  25 

—  granite,  20 

—  intrusive  forms  of,  27 

—  laccolith,  131 

mineralogical  composition,  21 

obsidian,  25 

occurrence,  27 

phenocrysts,  19 

plutonic,  17 

porphyritic,  24 

pumice,  26,  34 

sills,  29 

stocks,  31 

syenite,  22 

texture  of,  18 

—  volcanic,  32,  33,  34 

volcanic  necks,  30 

Incised  meanders,  138 
Inherited  soils,  103 
Intrusive  rocks,  27 
Iron  minerals,  hematite,  10 
limonite,  10 


SUBJECT  INDEX 


315 


Iron  minerals,  magnetite,  11 

—  oxides,  70 

—  pyrite,  11 
siderite,  11 


Joint  systems,  56 
Jurassic  period,  293,  296 

K 

Kainite,  10  * 
Kames,  217 
Kaolin,  basic  mineral  of  shales,  41 

—  chemical  composition,  42 

—  formula,  14 
Kaolinite,  14 
Kelp,  271 

Knox  dolomite,  87 


Labradorite,  13 

Laccoliths,  31 

Lacustrine  soils,  248 

Lafayette  formation,  143,  186,  267 

origin  of,  260 

references  on,  269 

Lagoons,  260 

-  filling  of,  260 

Lake,  currents,  240,  241,  242 

—  deposits,  246 

—  filling,  by  deltas,  245 

by  vegetation,  253,  254,  255 

—  shore  lines  at  different  water  levels, 

242,  243 
Lakes,  barrier  beaches,  242 

—  coastal  plain,  239 

—  delta,  167,  239 

—  deposits  and  basins,  246,  255 

—  effects  of,  240 

—  extinct,  224,  247,  249 

—  glacial,  223,  238 

-  kinds  of,  236 

—  oxbow,  158 

—  references  on,  258 

—  river,  239 

—  saline,  248 

—  shore  regions  of,  240 

—  sink-hole,  123 


Lakes,  soils  made  by,  248 

—  topography  of  bottoms  of,  247 

—  waves  on,  241 
Laminae  defined,  45 
Landslides,  188 

Panama  canal,  181 
Lapille,  33 
Lava,  32 
Leucite,  281 
Limestone,  42,  296 

—  as  fertilizer,  284 

—  chemical  composition  of,  44 

—  coquina,  43 

—  corals,  43 

—  microphotograph  of,  55 

—  soils,  85 

—  varieties,  44 
Limonite,  10,  128 

—  as  iron  ore,  70 
Loess,  and  glaciation,  234 

—  main  soils,  118 

—  mineralogical  composition,  114 

—  origin,  114 

—  productiveness  of,  117 

—  properties  and  occurrence,  112 

—  references  on,  234 
Luster,  in  minerals,  6 

M 

Magnetite,  11, 
Mantle  Rock,  16 
Marble,  55 

—  microphotograph  of,  55 

—  uses,  56 
Marine  deposits,  269 

—  marshes,  262 
Marl,  255 

Marshall  silt  loam,  118 
Meanders,  incised,  138 

—  flood  plain,  157 

—  deposition  by,  159 

—  development  of,  158 
Memphis  silt  loam,  118 
Mesozoic  era,  296,  293 
Metamorphic  rocks,  gneiss,  52 
kinds  of,  52 

marble,  55 

origin,  46 


316 


SUBJECT  INDEX 


Metamorphic  rocks,  quartzite,  54 
schists,  52 

—  slate,  53 

slaty  cleavage   and  schistosity  of, 

49 
Metamorphism,  agents  of,  47 

—  changes  produced  by,  47 

—  contact,  50 

—  defined,  46 

—  regional,  51 
Miami  silt  loam,  118 
Micas,  13 

—  formed  from  orthoclase,  49 

—  in  cecil  soils,  98 

—  in  schist  soils,  101 
Mineral  fertilizers,  273 

gypsum,  284 

limestone,  284 

—  nitrates,  283 

phosphates,  273 

potash,  281 

references,  284 

veins,  124 

Minerals,  ferro  magnesian,  15 

—  general  characters  of,  6 

—  in  loessial  soils,  114 

—  iron,  10 

—  relative  percentages  in  earth's  crust,  5 

—  silica  and  the  silicates,  11 

—  soil  and  rock  making,  8 
Mirabilite,  10 

Mississippi  period,  293,  295 
Monoclinal    structure    of    sedimentary 

rocks,  45 
Moraines,  203 

—  ground,  204,  206 

—  recessional,  205 
-  terminal,  204 
Muscovite,  13 


N 


Nitrates,  283 
Nitre,  10 


Obsidian,  25 
Oceans,  259 
Oligoclase,  13 


Olivine,  13 

Orangeburg  soils,  71,  127 
Organisms,  relation  to  weathering,  80 
Ordovician  rocks,  276 

—  period,  294 
Orthoclase,  12 

—  as  potash  fertilizer,  281 

—  hardness  of,  6 
Outwash  plains,  213 
Ox-bow  lakes,  158 

Oxidation  as  a  weathering  process,  69 


Paleozoic  era,  294 

Peat,  deposits,  nature  of,  255 

—  in  coal  formation,  295 
Pegmatite,  21 
Pennsylvanian  period,  295 
Penn  series,  93 

Permian  period,  165,  236,  293,  296 
Phenocrysts,  19 
Phosphates,  273 

—  bedded  rock,  276 

-  Florida,  278,  297 

—  kinds,  273 

—  origin,  274 

—  producing  regions  of,  275 

—  references,  284 

—  residual,  275 
Phosphate-bearing  rocks,  273 
Phosphatic  limestones,  44 
Piedmont,  46 

—  diorite,  99 

—  gneiss  as  soil  former  in,  52 

-  in  N.  C.,  98 

—  in  Pa.,  92 

—  plateau,  266,  286,  293 

—  'region,  46,  52,  92,  105,  166 
Pitchstone,  26 

Plant  foods  in  loessial  soils,  117 
Plants,  relation  to  weathering,  79 
Plucking  by  glaciers,  197 
Plutonic  rocks,  17 
Porphyritic  texture,  24 
Potash,  281 

—  from  Strassfurt  region,  282 
Pre-cambrian  era,  293 


SUBJECT  INDEX 


317 


Pressure  as  a  metamorphic  agent,  48 
Pumice,  26,  34 
Pyrite,  11 

Q 

Quaternary  period,  293,  297,  298 
Quartz,  essential  minerals  in  granite,  20 
—  hardness  of,  6 

—  percentage  in  igneous  rocks,  17 
Quartzite,  54 

R 

Residual  soils,  chemical  composition,  88 

defined,  68 

from  basic  rocks,  100 

granites,  97 

-  limestone,  85,  86,  87,  88 

quartzite,  91 

sandstone,  91 

—  schists,  101 

—  shale,  92 

—  slate,  93 
Rivers,  see  Streams 
Rocks,  16-65 

— •  classification  of,  16 

—  clastic,  37 

—  igneous,  17 

—  mantle,  16 

—  metamorphic,  46 

—  references  on,  65 

—  sedimentary,  37 

—  weathering  of,  67 


Salt,  see  Halite. 
Saltpeter,  10 
Sandstones,  39,  295 

—  chemical  composition,  40 

—  weathering  of,  90 
Saturation  in  soils,  80 
Schistosity,  49 
Schists,  52 

Scoria,  34 

Sea  Islands,  262 

Sedimentary  rocks,  see  Clastic  rocks. 

Selenite,  9 

Serpentine,  15 


Shales,  41 

—  chemical  composition,  42 
Sharkey  clay,  154 

Shore  currents,  241,  242 

—  lines  at  different  water  levels,  242 
Siderite,  11 

Silica  and  the  Silicates,  11-15 

—  Augite,  13 

—  Feldspars,  12,  13 

—  Ferromagnesian  minerals,  15 

—  Glauconite,  14 

—  Hornblende,  13 

—  Kaolinite,  14 

—  Micas,  13 

—  Olivine,  13 

—  Orthoclase,  12 

—  Serpentine,  15 

—  Talc,  14 

—  Zeolites,  14 
Sills,  29 

Silurian  period,  294 

Sink  holes  due  to  solution,  123 

Sirocco  the,  107 

Slates,  origin  of,  53 

Soil,  classes,  29 

—  creep,  181,  189 

—  important  soil-making  minerals,  8 

—  regions  of  U.  S.,  285 

—  series,  29 

—  type,  29 

Soil  series  from  granites,  Cecil,  98 

Chester,    98 

Durham,  98 

—  Iredell,  100 
Soil  water,  124 
capillarity,  125 

—  chemical  work  of,  126 

—  carbonation,  127 

—  deposition,  127 

oxidation,  126 

solution,  127 

—  defined,  124 

mechanical  work  of,  126 

—  movements  of,  125 

—  references  on,  130 

terraces,  161 

Soils,  alluvial,  133 

—  basalt,  100 


318 


SUBJECT  INDEX 


Soils,  colluvial,  186 

—  diorite,  99 

—  eolian,  107 

—  glacial,  209 

—  granite  and  gneiss,  95 

—  inherited,  103 

—  lake  beach,  243 

—  limestone  and  marble,  85 
-loessial,  114,  117 

—  obsidian,  101 

—  of  deltas,  176 

flood  plains,  153 

moraines,  206,  207 

—  quartzite,  91 

—  sandstone,  90 

—  schist,  101 

—  shale,  92 

—  slate,  93 

Solution  as  a  weathering  process,  71 

Specific  gravity  of  minerals,  8 

Springs,  131 

Stocks,  31 

Stratification,  bedding  planes  of,  45 

—  definition  and  cause  of,  45 

—  laminae,  45 

—  monoclinical  structure  of,  45 
Stratum,  defined,  45 

Streak  of  minerals,  6 
Streams,  abrasion  by,  146 

—  aggrading,  139 

—  alluvial  deposits,  148-164 

—  alluvial  soils,  165 

—  analyses  of  waters,  135 

—  canyons,  136 

—  corrasion  by,  135 

—  corrosion  by,  134 

—  currents,  145 

—  curves  in,  138 

—  degrading,  139 

—  deltas,  166-171 

—  deposition  by,  150,  167 

—  dissolved  material,  134,  135 

—  drainage  and  sediment,  146 
lines,  137 

—  erosion  by,  134,  140 
—  of  banks,  138 

-flood  plains,  151,  152,  156,  157 
settlement  of,  159 


Streams,  flood,  soils  of,  153 

—  graded,  139 

—  levees,  151,  152 

—  meandering  of,  138,  139,  140 

—  organization,  133 

—  references  on,  177,  178 

— •  size  of  materials  carried  by,  145,  147 

—  slope  of,  139 

—  sources,  133 

—  terraces,  161,  162 
—  soils,  163,  164 

—  transporting  power  of,  143,  145,  149 

-  valleys,  137,  138,  156 

-  velocity  of,  134,  145,  148 

—  work,  134 
Strike,  defined,  58 

—  relation  to  ice  movement,  211 
Swamps,  250 

—  classes  of,  251 

—  alluvial,  251 
coastal  plain,  252 

—  glacial,  251 

—  deposits,  255 

—  factors  in  formation  of,  250 

-  filling  of,  253 

—  reclamation  of,  251 

—  references  on,  258 
Syneline  defined,  59 

Synenite,  chemical  composition  of,  22 

—  mineralogical  composition,  22 


Talus,  187 

Temperature  changes,  relation  to  weath- 
ering, 73 

Tenacity  of  minerals,  7 
Terrace,  wave-cut,  241 

—  wave-built,  241 
Tertiary  period,  36,  293,  297 
Texture,  felsitic,  25 

—  glassy,  19 

—  granitoid,  18,  20 

—  of  igneous  rock,  18 

—  porphyritic,  19,  24 
Throw  defined,  64 
Tides,  259 

Tishomingo  formation,  104 
Trenton  limestone,  211 


SUBJECT  INDEX 


319 


Triassic  period,  28,  93,  293,  296 
Trilobite,  294 
Trona,  10 

U 
Underground  water,  see  Ground  water. 


Valleys,  drowned,  263 

—  formation  by  rivers,  137 

—  hanging,  221 
Valley  train,  216 
Veins  of  minerals,  124 
Volcanic  necks,  30 
Volcanoes,  32 

—  ejecta  from,  32 

—  fissure  flows  from,  36 

—  types  of  eruptions,  35 
Volusia  soils,  210 
Vulcanism,  32 

W 

Wabash  clay,  154 

Water,  ground,  see  Ground  water. 

Water  table  defined,  120 

Waves,  barrier  beach  by,  242,  260 

—  cliff  cut  by,  241 

—  shore  currents,  242 


Waves,  terrace  cut  by,  241 
—  built  by,  241 

—  undertow,  241 
Weathering,  67 

—  carbonation,  68,  127 

—  exfoliation,  75 

—  freezing,  76 

—  gravity,  77 

—  humification,  78 

—  hydration,  70 

—  oxidation,  69,  126 
-  rate  of,  88 

—  solution,  71,  127 

—  temperature  changes,  73 
Wells,  131 

—  references  on,  132 
Wind  work,  abrasion,  110 

—  dunes,  109 

—  importance  of,  107 

—  references  on,  119 

—  the  loess  and,  112 

—  transportation  by,  108 


Yazoo  loam,  154 


Zedolites,  14 


Wiley  Special  Subject  Catalogues 

For  convenience  a  list  of  the  Wiley  Special  Subject 
Catalogues,  envelope  size,  has  been  printed.  These 
are  arranged  in  groups — each  catalogue  having  a  key 
symbol.  (See  special  Subject  List  Below).  To 
obtain  any  of  these  catalogues,  send  a  postal  using 
the  key  symbols  of  the  Catalogues  desired. 


1— Agriculture.     Animal  Husbandry.    Dairying.     Industrial 
Canning  and  Preserving. 

2 — Architecture.       Building.      Masonry. 

3 — Business  Administration  and  Management.     Law. 

Industrial  Processes :   Canning  and  Preserving;    Oil  and  Gas 
Production;  Paint;  Printing;  Sugar  Manufacture;  Textile. 

CHEMISTRY 

4a  General;  Analytical,  Qualitative  and  Quantitative;  Inorganic; 
Organic. 

4b  Electro-  and  Physical;  Food  and  Water;  Industrial;  Medical 
and  Pharmaceutical;  Sugar. 

CIVIL  ENGINEERING 

5a  Unclassified  and  Structural  Engineering. 

5b  Materials  and  Mechanics  of  Construction,  including;  Cement 
and  Concrete;  Excavation  and  Earthwork;  Foundations; 
Masonry. 

5c  Railroads;  Surveying. 

5d  Dams;  Hydraulic  Engineering;  Pumping  and  Hydraulics;  Irri- 
gation Engineering;  River  and  Harbor  Engineering;  Water 

Supply. 

(Over) 


CIVIL  ENGINEERING—  Continued 

Se  Highways;  Municipal  Engineering;  Sanitary  Engineering; 
Water  Supply.  Forestry.  Horticulture,  Botany  and 
Landscape  Gardening. 


6 — Design.       Decoration.       Drawing:     General;      Descriptive 
Geometry;  Kinematics;  Mechanical. 

ELECTRICAL  ENGINEERING— PHYSICS 
7 — General  and  Unclassified;  Batteries;  Central  Station  Practice; 
Distribution  and  Transmission;  Dynamo-Electro  Machinery; 
Electro-Chemistry  and   Metallurgy;   Measuring     Instruments 
and  Miscellaneous  Apparatus. 


8 — Astronomy.      Meteorology.      Explosives.      Marine    and 
Naval  Engineering.     Military.     Miscellaneous  Books. 

MATHEMATICS 

9 — General;    Algebra;   Analytic  and   Plane   Geometry;    Calculus; 
Trigonometry;  Vector  Analysis. 

MECHANICAL  ENGINEERING 

lOa  General  and  Unclassified;  Foundry  Practice;  Shop  Practice. 
lOb  Gas  Power  and    Internal   Combustion  Engines;  Heating  and 

Ventilation;  Refrigeration. 
lOc  Machine  Design  and  Mechanism;  Power  Transmission;  Steam 

Power  and  Power  Plants;  Thermodynamics  and  Heat  Power. 
11 — Mechanics.  

12 — Medicine.  Pharmacy.  Medical  and  Pharmaceutical  Chem- 
istry. Sanitary  Science  and  Engineering.  Bacteriology  and 
Biology. 

MINING  ENGINEERING 

13 — General;  Assaying;  Excavation,  Earthwork,  Tunneling,  Etc.; 
Explosives;  Geology;  Metallurgy;  Mineralogy;  Prospecting; 
Ventilation. 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 
BERKELEY  %^ 

Return  to  desk  from  which  borrowed. 
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REC'D  LD 

MAR -21 1962 


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g£0  10  to* 


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YC  21349 


